One of two Ruhmkorff Coils made by Queen & Co. The output with about 650 Watts of current in the primary was a torrent of sparks 45 inches in length.

INDUCTION COILS
How to Make, Use, and Repair Them

INCLUDING

RUHMKORFF, TESLA, AND MEDICAL COILS,
ROENTGEN RADIOGRAPHY, WIRELESS
TELEGRAPHY, AND PRACTICAL INFORMATION
ON PRIMARY AND
SECONDARY BATTERY

BY
H. S. NORRIE
(NORMAN H. SCHNEIDER)

SECOND EDITION, REVISED AND MUCH ENLARGED

NEW YORK:
SPON & CHAMBERLAIN, 12 Cortlandt St.

LONDON:
E. & F. N. SPON, Ltd., 125 Strand
1901

Entered, according to Act of Congress, in the year 1896
Re-entered for Copyright in 1901
By SPON & CHAMBERLAIN
in the office of the Librarian of Congress, Washington, D. C.

BURR PRINTING HOUSE,
NEW YORK, N. Y., U. S. A.

ERRATA.

Page 55. Should read, "Cords being attached to binding posts Nos. 1 and 2 are in circuit with the Secondary Coil only. When at Nos. 2 and 3 they receive the induced current or extra current in the Primary."

Index. "Tesla coil, descriptive," should read "Tesla coil, disruptive."

PREFACE TO THE SECOND EDITION.

The great favor with which the first edition of this little work has been received and the steadily growing interest in its subject, together with many valuable improvements and researches, may be given as the reasons for this new edition.

The book has been thoroughly revised, partly rewritten, and considerable new matter, with twenty-six new illustrations, added. It has been brought up to date as far as electrical science has gone.

To detail all that has been done is too great a task for a preface; we may briefly mention the following new matter:

Coils for gas and automobile engines; medical coils, concise directions for operation and repairs; new forms of contact breakers, including electrolytic and mechanical; gas-lighting apparatus; primary and secondary batteries.

The chapter on X-Ray Apparatus has been entirely rewritten, and is thoroughly practical; and an entire chapter on Wireless Telegraphy has been added. In a book of this size it is not feasible to give specific directions and full dimensions for the manufacture of all the apparatus described. Indeed, much of the latter must be adapted to the particular purpose for which it is to be utilized. Again, the same amount of material will not always produce the same results. A little closer winding, greater pressure applied to the cooling wax of a condenser, and the output or capacity of either is changed.

Matters purely of design or taste are to be governed by the creative faculty of the worker; but such general details and rules are given as will be sufficient to enable one possessing ordinary constructive ability to make his own apparatus.

The whole process of coil-making does not require high mechanical skill, but chiefly patience and attention to details; and, perhaps best of all, but few tools are needed, all of a simple kind.

We beg to acknowledge courtesies received from Messrs. Queen & Co., the Scientific American for frontispiece and Fig. 13, Mr. Goldingham's book on Oil Engines for Fig. 12, and others who have been of assistance to the author. The best American and English practice has been adopted; the American standard gauges and sizes of wires are used, except where noted.

A list of works, particularly of value to the coil worker, will be found following the index.

H. S. Norrie
(Norman H. Schneider.)

April, 1901.

CONTENTS.

CONTENTS OF TABLES.

LIST OF ILLUSTRATIONS.

Frontispiece, The Queen 45″ Spark Coil.

CHAPTER I.
COIL CONSTRUCTION.

In commencing a description of the Ruhmkorff coil and its uses, a brief mention of the fundamental laws of induction directly bearing on its action will assist in obtaining an intelligent conception of the proper manner in which it should be constructed and handled.

Any variation or cessation of a current of electricity flowing in one conductor will induce a momentary current in an adjacent conductor; and if the second conductor be an insulated wire coiled around the first conductor, also a coil of insulated wire, the effect is heightened. The intensity of the secondary or induced current increases with the number of turns of its conductor, the abruptness and completeness of the variation of current in the first or primary coil, and the proximity of the coils. And the insertion of a mass of soft iron within the primary coil by its consequent magnetization and demagnetization augments still further the inductive effect. There are other contributing causes which cannot be treated of here, but are of not so much importance as the foregoing.

In the Ruhmkorff coil, which is an application of the above laws, the primary coil is of large wire and the secondary coil of extremely fine wire, of a length many thousand times greater than the wire of the primary coil. The current is abruptly broken in the primary circuit by a suitable device—the contact breaker or rheotome. The current induced in the secondary at the make of the circuit is in the opposite direction to that of the primary coil and battery, but the current at the break of the circuit is in the same direction as that of the primary. The effect of the current at the break of the circuit is more powerful than that at the make, which latter is also somewhat neutralized by the opposing battery current. A condenser or Leyden jar is connected across the contact breaker to absorb an extra current induced in the primary coil by the break of the circuit, which would tend to prolong the magnetization of the core beyond the desired limit.

The whole apparatus is mounted on a wood base, having the condenser in a false bottom for the sake of compactness.

It is not herein intended to describe all the minor operations in the construction of a Ruhmkorff coil. A sufficient description and review of the main points to be considered, however, will be given to enable a person fairly proficient in the use of simple tools to construct a serviceable instrument.

The parts and their arrangement in relation to one another are shown in Fig. 1, but are not drawn strictly to scale, although very nearly so.

Fig. 1.

C is the core, consisting of a bundle of soft iron wires as fine as can be obtained. The greater the subdivision of the core the quicker will it respond to the magnetizing current in the primary coil, and lose its magnetism when the current ceases. It has another advantage, in that the disadvantageous eddy, or Foucault currents, are lessened, which fact, however, is of not enough importance to need extended consideration.

Many coil-makers saturate the core with paraffin or shellac, which is of slight benefit. This core is wrapped in an insulating layer of paraffined paper or enclosed in a rubber shell, there not being any great necessity to use more than ordinary insulation between the core and the primary coil.

In the majority of induction coils or "transformers" used in the alternating current system of electric lighting, the iron cores form a closed magnetic circuit. A closed magnetic circuit in a Ruhmkorff coil could be obtained by extending the iron core at each end and then bending and securing the ends together, forming, as it were, a ring partly inside and partly outside the coil. But although the inductive effects would be heightened and less battery power required, the slowness of the circuit to demagnetize would alone be detrimental to rapid oscillations of current.

There would also be a loss from a greater hysteresis (energy lost in the magnetization and demagnetization of iron). A core magnetizes quicker than it demagnetizes, and the latter is rarely complete; a certain amount of residual magnetism remains, hysteresis being strictly due to this retention of energy (Sprague). Hysteresis shows itself in heat, but must not be confounded with Foucault or eddy currents. The latter are corrected by subdividing the metal, but the former depends upon the quality of the metal, and increases with its length.

Moreover, a coil with a closed magnetic circuit requires an independent contact breaker.

In most of the alternating currents used in lighting their rapidity of alternation is but one hundred and twenty-five periods per second. As in the simple electromagnet, the proportions of diameter and length of the primary coil and core will determine its rapidity of action. A short fat coil and core will act much quicker than a long thin one. But on a short fat coil the outside turns would be too far removed from the intensest part of the primary field. A good proportion of core length is given in the following table:

The primary coil P consists of two or not more than three layers of insulated copper wire of large diameter, being required to carry a heavy current in a 2-inch spark coil, probably from 8 to 10 amperes. In designing the primary coil a great advantage arises from using comparatively few turns but of large wire. Each turn of wire in the primary has a choking effect upon its neighbor by what is termed self-induction.

As the primary coil and core may be considered as an electro magnet, it may not be out of place to notice the rule governing such. Magnetization of an iron core is mainly dependent upon the ampere turns of the coil surrounding it—that is, one ampere carried around the core for one hundred turns (100 ampere-turns) would equal in effect ten amperes flowing through ten turns. Practically speaking, there would be certain variations to the rule, for one difficulty would arise in that the smaller wire used in conveying the smaller current would fit more compactly and allow more turns to be nearer the core, the active effect of the turns always decreasing with their distance from the core. And although a large current and few turns would not have so much self-induction, there would be trouble at the contact breaker, owing to the large current it would have to control.

The most suitable sizes of wire for the primary coil are: No. 16 B. & S. for coils up to 1 inch spark; No. 14 B. & S. up to 4 inches of spark, and No. 12 B. & S. for a 6-inch spark coil. The coil should be, say, one-twelfth of the core length shorter than the core.

I is the insulating tube between the primary coil and the secondary coil S. Here great precaution is necessary to prevent any liability of short circuiting or breaking through of sparks from the secondary coil. This danger cannot be underestimated, and the tube should be either of glass or hard rubber, free from flaws, varying in thickness with the dimensions of the coil. It should extend at least one-tenth of the total length of the primary coil beyond it at each end. The end of this tube can be turned down so as to allow of the hard rubber reel ends being slipped on and held in position by outside hard rubber rings (Fig. 2).

Fig. 2.

The secondary coil consists of many turns of fine insulated copper wire separated from the primary coil by the insulating tube and a liberal amount of insulating compound at each end. In coils giving under 1 inch of spark this coil may be wound in two or more sections.

Fig. 3.

The usual manner of constructing these sections is to divide up the space on the insulating tube by means of hard rubber rings placed at equal distances apart, in number according to the number of sections desired (Fig. 3). The space between each set of rings, or between the coil end and a ring, is wound with the wire selected, the filled sections constituting a number of complete coils, which are finally connected in series. The sectional method of winding prevents the liability of the spark jumping through a short circuit, but heightens its tendency to pass into the primary coil at the ends, where it must be therefore specially insulated from it.

Fig. 4.

In winding these sections there is a method now generally adopted which has many good points, although at first it may seem complicated. The old way of filling two sections was to wind both in the same direction as full as desired, then join the outside end of the left-hand coil to the inside end of the right-hand coil. This necessitated bringing the outside end down between two disks, or in a vertical hole in the sectional divider, and thereby rendered it liable to spark through into its own coil. This is shown in Fig. 4, A and C inside ends, B and D outside ends, the disk being between B and C.

Fig. 5.

Reference to Fig. 3 shows the new method, and Fig. 5 shows an enlarged diagram of sections 2 and 3 of Fig. 3.

Sections 1 and 3, Fig. 3, are filled with as many turns as desired; the spool is then turned end for end, and sections 2 and 4 are wound, being thus in the opposite direction of winding to sections 1 and 3.

The inside ends of 1 and 2 and 3 and 4 are soldered together, and the outside ends of 2 and 3 are also soldered together.

The outside ends of 1 and 4 serve as terminals for the coil.

This method of connection leaves all the turns so joined that the current circulates in the same direction through them all, as will be seen by an examination of the enlarged diagram, Fig. 5.

Sprague, in his "Electricity: Its Theory, Sources, and Application," recommends that the turns of wire in the secondary coil shall gradually increase in number until the middle of the spool is reached, and then decrease to the spool end, in order that the greatest number of turns be in the strongest part of the magnetic field (see Fig. 6). D D D are section dividers, S secondary windings, P primary coil. The selection of the size of wire to be used depends on the requirements as to the spark. If a short thick spark be desired, use a thick wire, say No. 34 B. & S.; if a long thin one, use No. 36 to No. 40 B. & S.

Fig. 6.

Although it is impossible to lay down rules for determining the exact amount of wire to be used to obtain a certain sized spark, yet a fair average is to allow 1¼ pounds No. 36 B. & S. per inch spark for small coils and slightly less for large ones.

The most satisfactory and perhaps the easiest way for large coils is to wind the secondary in separate coils, made in a manner similar to that employed in winding coils for the Thompson reflecting galvanometer. This method, first described by Mr. F. C. Alsop in his treatise on "Induction Coils," is somewhat as follows:

A special piece of apparatus (Figs. 7 and 8) is necessary, but presents no great difficulty in manufacture. A metal disk, D, one-sixth of an inch thick and 7 inches in diameter, is mounted on the shaft S. A second disk is provided with a collar and set screw, A, in order that it may be adjusted on the shaft at any desired distance from the stationary one. When the diameter of the coil to be wound has been decided upon, a wooden collar, W, with a bevelled surface is slipped on the shaft, it corresponding in diameter with the desired diameter of the hole through the centre of the secondary coil. As these coils are going to be made as flat rings and slipped on over the insulating tube, a remark here becomes necessary on this diameter. Reference to Fig. 9 will show that it is intended that the coils near the reel ends shall fit very loosely on the tube T (Fig. 1)—in fact, that there shall be a clearance of possibly one-half inch in the extreme end, diminishing gradually to a fifteenth of an inch in the centre coils. Therefore it becomes necessary to provide a number of wooden rings equal to the desired diameter of the central hole in the coil. The thickness of the wood determining the width of the individual coil depends on the selection of the operator; but the rule may be laid down that the narrower the coils the better the insulation of the complete coil will be on completion.

Fig. 7. Fig. 8.

Fig. 9.

One-sixteenth of an inch is a very fair average, and has been generally adopted by the writer.

A quantity of paper rings are now cut out of stout writing paper which has been soaked in melted paraffin. If a block or pad of letter paper be soaked in paraffin and allowed to become cold under pressure, the ring may be scratched on the surface of it and the block cut through on a jig saw. The central apertures of course will vary in size with their position on the tube T (Fig. 9).

The coil winder is now either mounted in a lathe or fixed in a hand magnet winder in such manner that it can be steadily and rapidly rotated. The wire to be wound comes on spools, which can be so threaded on a piece of metal rod that they turn readily. A metal dish containing melted paraffin is provided with a round rod, preferably of glass, fixed under the paraffin surface, so that it can rotate freely when the wire passes under it through the paraffin. Two paper rings are slipped on the winder that they may form, as it were, reel ends for the coil, and if the metal disks have been warmed it is an easy matter to lay them flat.

The end of the wire is then passed through the paraffin under the glass rod and through the hole H in the metal disk for a distance of, say, 6 inches, and held to the disk outside with a dab of paraffin or beeswax. Then the winder is rotated and the space between the paper disks is filled with wire. The paraffin, being hot, will adhere to the wire, and cooling as the wire lays down on the winder, hold the turns together and at the same time insulate them from each other. It will not be possible to lay the wire in even layers, as would be necessary in winding a wider coil, but the spaces can be filled up, using ordinary care that no radical irregularity occurs—that is, that only adjacent layers are likely to commingle.

When the space is filled up to the level of the paper disks and the paraffin is hard, loosen the set screw, and removing the outside disk, the coil can be slipped off, or a slight warming will loosen it. Any number of these coils can be made, and there are the advantages in this mode of construction that a bad coil will not spoil the whole secondary, and that the wire can be obtained in comparatively small quantities.

As each coil will not be of very high resistance, the continuity of the wire can be readily tested by means of a few cells of battery, connecting one end of the coil to one pole of the battery, and the other pole of the battery and coil end touched to the tongue. If a burning sensation is experienced, the connection is not broken. Where possible the coils should be measured as to their resistance on a Wheatstone bridge.

When the requisite number of coils has been prepared, they are assembled in the following manner (Fig. 9): The coils, having their aperture diameter graded, are placed in order, and starting with the one having the largest hole, it is slipped over the primary protection tube T, one end being brought out through a hole in the reel end drilled vertically or between the reel end and the coil. A couple of paper rings are then slipped on the tube, and another coil placed over them, having its ends connected as in Fig. 3. This process is continued until all the coils are in place. The annular space between the coils and the tube T (Fig. 9) is filled in with melted paraffin and the coils gently pressed together, so as to form a compact mass, paraffin being poured over the outside of the whole combination. Before winding any wire used in this work it must be perfectly dry, which end can be accomplished by subjecting the whole spool to a short period of baking in a moderately warm oven.

The accompanying table gives the length of No. 36 silk-covered wire that will fill a linear space equal to one thickness of the wire in different-sized rings. This size wire wound tight will give 125 turns per linear inch. Therefore on a ring having a middle aperture of 1½ inches and an outside diameter of 4 inches, there will be 156 turns, or a total length of 1347 inches. This is obtained as follows: 1½ inches × 3.1416 = 4.7124 (or 4.712); 4 inches × 3.1416 = 12.5664 (or 12.56); (4.712 + 12.56)∕2 = mean circumference—viz., 8.635 inches.

This mean × number of turns in thickness of ring between the two circumferences—viz., 156 = 1347 inches.

Table of Secondary Windings.

To obtain the length of wire necessary for a ring occupying more than the space of one turn on the primary insulating tube, multiply the length before obtained by the number of turns in the space it occupies. Thus a flat ring one-tenth of an inch thick would equal 1347 inches × 12.5.

This rule is necessarily only approximate, owing to the way the wires bed on each other from their cylindrical section. In actual practice, when the wire is run through the paraffin bath not more than 50 per cent of the calculated wire will occupy the space. And the thickness of the paper rings must also be added when figuring the total length of the coil. In the iron-clad transformers or induction coils of highest efficiency used in the alternating current electric light system, the rule for determining the windings of the coils is based on the ratio of the turns of wire in the primary to the turns in the secondary, the electromotive force in the primary, and the lines of force cut by the windings.

The secondary ends can be attached to binding posts mounted on the reel ends. Unless these reel ends be very high and clear the outside of the coil considerably, it is better to mount the binding posts on the top of the hard rubber pillars. A neat plan is to mount on the top of the coil a hard rubber plate reaching from reel end to reel end, and place the binding posts on that.

A discharger consists of two sliding metal rods with insulated handles passing through pillars attached to the secondary coil. The inside ends of these rods is provided with screw threads for the ready attachment of the balls, points, etc., which are to be used. The substance to be acted upon is laid on a rubber or glass table midway between the rod pillars and slightly below the level of the rods.

By hinging the rod pillars, or using a ball and socket joint, the discharger can be inclined so as to be better brought near the substance on the table.

The next important part of the coil is the contact breaker.

The armature R is a piece of soft iron carried at the end of a stiff spring, in about the middle of which, at B, is riveted a small platinum disk or stud. The adjusting screw A has its point also furnished with a piece of platinum, which is intended to touch the platinum on the spring when the latter is in its normal position. The core C of the coil serves as an electro-magnet. When the current flows from the battery (represented by the figure at L) through the primary coil and armature spring to the adjusting screw, it causes the armature to be drawn to the magnetized core, but thereby draws the platinum disk away from the adjusting screw. In so doing it breaks the circuit, the magnet loses its power, and the elasticity of the spring reasserting itself, carries the armature back, thereby reclosing the circuit. This is repeated many times in a second, the result being a continual vibration of the spring, and a consequent interruption to the current.

The condenser or Leyden jar J, connected as in the diagram to the base of the vibrating spring at K and to the adjusting screw wire M, is constructed as follows: On a sheet of insulated paper is laid a smaller sheet of tinfoil, one edge of which projects an inch or so over one end of the paper. Another sheet of paper covering this carries a second sheet of tinfoil, one end of which projects as in the first sheet, but at the opposite end of the paper. Tinfoil and paper sheets are laid in this manner alternately until a sufficient number is attained. The projecting ends are then clamped together and the whole pile immersed in melted paraffin, as will be described in a subsequent chapter. Wires are affixed to these clamped ends which serve to connect the condenser with the contact breaker. The conventional sign for a condenser is that used at J, showing the two series of plates, the insulation or dielectric, as it is called, being understood.

The size of condenser to use with different-sized coils varies according to the winding of the primary and the battery used. A primary coil of few turns would not necessitate as large a condenser as one of a large number of turns. At the same time, a condenser may be made of too great a capacity, and thereby weaken the action of the coil.

The base upon which the coil and its parts are mounted may be of dried polished wood. But where the coil is designed to give large sparks—over 2 inches—it is an advantage to use hard rubber one quarter of an inch and upward in thickness. Glass, were it not for the difficulty of drilling it and its brittleness, would be a desirable material for a coil base in a dry atmosphere. Hard red or black fibre coated with shellac varnish is also serviceable, and, moreover, is extremely easy to work. Slate must never be used; there is too much liability of iron veins being found in it, which in such high tension experiments as will be described would seriously impair the usefulness of the apparatus. The material selected for the base must be one that will not absorb moisture. A paraffined surface collects moisture up to a certain point in isolated drops, whereas a glass and even a hard rubber surface condenses the moisture as a film, which latter is extremely undesirable. But unfortunately the fact that a paraffined surface does not present a pleasing appearance would probably result in its rejection. And lastly, by mounting the coil on hard rubber blocks, or extending the reel ends to raise the coil body, a high insulation can be obtained at the sacrifice perhaps of appearance or height. From the care taken to insulate the secondary coil, it may be considered a superfluous precaution to so carefully select a base, but practical work with the instrument at some important crisis will demonstrate the necessity of extreme care in the smallest details relating to insulation. It may be well to note here that hard rubber is acted upon by ozone, and is thereby impaired as an insulator.

Fig. 10.

The base forms the top of a flat box in which the condenser lies; but there are a few points worth considering right here. As the connections of the coil will probably be under the base, a sufficient space must intervene between the base and the top of the condenser. It is a good plan to lay the condenser at least one half inch below the top of this box, and fill up to, say, one eighth of an inch with melted paraffin, leaving the condenser wires projecting for attachment. The connections of the primary coil and contact breaker should by all means be soldered, not simply wires held under screw nuts. And, moreover, all wires under the base should be so run that they do not cross one another, which precaution only requires a little planning. Then, when the connections are all made and the base laid on top of the box, it can be pressed down if the paraffin be warm, so that the screw heads and wires mark out their own channels and cavities in which to lie.

A commutator or pole-changing switch is often added to change the polarity of the battery current. The diagram of connection is shown in Fig. 10. When the levers are as in the figure, the circuit is broken and no current flows through the coil.

Coils in Series.

Ruhmkorff coils can be connected in series, but it is not to be recommended. When it becomes necessary, however, the cores should be removed, and one long core inserted, extending through each primary. This will bring the time constants of each primary coil together and prevent the interference otherwise present. The primary coils and secondary coils are connected in series by assuming that they are but adjacent sections of one complete instrument. Of course, as the resistance of the primary is raised, the electromotive force of the battery must be raised also.

Oil Immersed Coil.

A highly satisfactory induction coil can be made without much labor and few tools, and will prove useful in many experiments which would not warrant a more expensive instrument.

Make a bundle of soft iron wires, No. 22 B W G, for the core, ten inches in length and one inch or more in diameter. Wrap this with insulating tape or even ordinary tape to prevent the primary coil from coming in contact with the iron. Now, wind on a primary of two layers No. 14 B & S gauge cotton-covered copper wire, and insert the coil into a hard rubber (or glass preferred) tube large enough to hold the coil tight and to project an inch or so beyond the core ends.

A secondary coil of about one pound No. 36 cotton-covered magnet wire should now be made on a hard rubber spool, the hole through centre of this spool must be at least one inch larger in diameter than the diameter of the primary cover. This spool should not exceed four inches in length, and is to be slipped over the primary coil and held suspended by blocks of wood in such a manner that it does not touch the primary coil or cover. The whole outfit is now immersed in an earthenware or glass vessel filled with linseed or heavy paraffin oil. The contact breaker and condenser will be mounted independently; the condenser for the two-inch spark coil will be suitable (see Table on [page—7]).

"Tesla" Coil.

The coil just described, without contact breaker or iron core, can be connected up and used in place of a "Tesla coil," which it resembles. The coils used by Nikola Tesla are so many and varied that it becomes a difficult task to describe a mode of construction which will meet the wants of those who ask for "Tesla" coils. The American Electrician gives a description of one wherein a glass battery jar, 6 inches × 8 inches, is wound with 60 to 80 turns of No. 18 B & S magnet wire. Into this is slipped a primary, consisting of 8 to 10 turns of No. 6 B & S wire, and the whole combination immersed in a vessel containing linseed or mineral oil.

Fig. 11.

Disruptive "Tesla" Coil.

For Fig. 11 the specification is as follows: Secondary, 300 turns of No. 30 B & S silk-covered magnet wire, wound on rubber tube or rod, and the ends encased in glass or rubber tubes. This is inserted into the primary, which consists of two coils, each of 20 turns No. 16 B & S rubber-covered wire, wound separately on a long rubber tube not less than ⅛ inch thick. The last tube must be large enough to be very loose when the secondary coil is inserted in it, and it must project at least two inches over each end of the secondary. A hard rubber division must be placed between these primary coils. The four ends of the latter coils are connected C C to two condensers and D D to two discharger balls, the secondary wires going to the exhibitive apparatus. A further description of these connections is to be found in [Chapter XII]., also notes upon the use of the disruptive coil.

Coils for Gas Engines.

These are either primary only or primary and secondary. Two to three pounds of No. 14 B & S magnet wire are wound on an iron wire core eight to ten inches in length by one inch in diameter. The contact is made and broken in the igniter of the engine as at the wipe spring of a ratchet gas burner. Four to eight large cells of dry battery are used, or eight cells Edison-Lalande—iron-clad type. Number of cells varies with size of coil needed, some classes of engines require a heavier spark than others to ignite the vapor.

When a primary and secondary are used, the primary should be made of two or three layers No. 14 B & S magnet wire, and a secondary of one pound No. 34 B & S magnet wire. There can be an independent contact breaker or the coil can be made up similar to a one-half inch spark Ruhmkorff coil (see Chapter I.).

Fig. 12.

The method of connecting up a coil of the latter description is shown in Fig. 12, which is self-explanatory. It shows a form of cam-shaft switch which is operated by the engine, and which opens and closes the primary circuit of the induction coil, the sparks from the secondary winding passing between the points of the igniter in the engine cylinder. As shown in Fig. 12, the igniter or ignition plug is similar in operation to a coil discharger, the two terminals being, however, insulated from each other by the use of porcelain. To ensure a good insulation under the severe working conditions has been somewhat of a task, but it seems to have been attained in the types of igniters known as the Splitdorf and the Roche or New Standard.

The Splitdorf gas-engine coil is the result of much experiment and careful design. It is built to stand hard usage, and the insulation used has been adopted only after exhaustive test. In automobile work, where a heavy strain is made upon the engine, as in climbing heavy grades, it has been found that a stronger spark gives surer results. This would indicate more battery current through the coil, and it is a wise precaution to have a few extra cells attached that can be switched on if necessary.

In constructing spark coils for gas engines particular care must be given to the contact breaker. In most types of gas or oil vapor engines it is absolutely necessary to have the spark pass with uniform regularity, and immediately and surely when required. For automobiles or where the apparatus is subject to jar, a heavy iron vibrating armature would become unreliable by reason of its inertia and its responding to shock. At every jolt of the vehicle it would jar and get out of rhythm, and it certainly seems preferable to use a mechanical contact apparatus whenever feasible. In the older type of gas engine the spark is made by mechanism breaking contact right in the vapor. The actual arrangement of these devices is detailed and illustrated in the later works on gas and oil engines.

Resistance Coils.

Although foreign to the title of this book, these coils will be mentioned, being often necessary as accessories to the operation of coils, wireless telegraphy, etc. These are coils of insulated German silver wire, wound to a specified resistance. The main feature about those designed for testing is that they are wound non-inductively—that is, the wire is wound double in such manner that the current flows both ways around the turns, and so neutralizes the inductive action. In cases where dynamo current is to be used, as in telegraphs operated from dynamo current, the coils are wound on tin tubes to make them fireproof and yet radiate the heat. As the resistance of German silver varies very largely, only approximate figures can be given. The table ([page 64]) has been made up from the best averages obtainable. The carrying capacity of resistance coils varies with their construction, the better they can radiate heat, the more current they can safely carry.

General Remarks on Coils, etc.

Ruhmkorff induction coils should always be fitted with a switch to open, close, or reverse the power circuit, a double throw, double pole, baby knife switch, mounted on a separate porcelain base, is very suitable. Such a switch is open when the handle is vertical, and it should always be left so when changing connections, fixing battery, etc. A large, well-finished coil will have the secondary wires brought in rubber tubes to binding posts mounted on hard rubber pillars, or to binding posts mounted considerably above the coil cover level. A very neat mode is shown in the frontispiece on the large 45-inch spark coil. Here the secondary wires go to hard rubber pillars, which also carry adjustable rod dischargers. These rods are movable towards or away from each other by means of the large hard rubber handle to which they are connected by a simple system of levers. In this coil the secondary is moulded on a flexible tube, which fits loosely over the primary tube in order to compensate for changes of temperature and consequent expansions and contractions. All well-designed coils should be so arranged that the primary coil and core can be readily removed from the secondary, or vice versa. It is sometimes desirable to use a different primary. This arrangement will greatly facilitate any necessary repairs. It must be always remembered that the working of a coil depends on the insulation between primary and secondary. Spare no pains to have perfect insulation; it is a hopeless task to reinsulate a broken-down secondary, although the sectional method of winding facilitates repairs. In large winding rooms it is customary to have a revolution counter connected to the spindle, so that the number of turns can be seen at all times. A bicycle cyclometer can be readily fitted up for this purpose, and will be found of considerable assistance where a number of sections are needed, each with a similar number of turns. In the commercial construction of telephone coils and magnet spools it is often the rule to specify only the number of turns of the requisite size wire, the ampere turns of the coils being thus regulated.

The Testing of a Coil for Polarity.

This is often necessary, and may be done in a variety of ways. When the coil is working, and sparks be passed between fine wires mounted on the discharger, the positive wire tip will be cold, whereas the negative end will be quite hot. In vacuo, the positive shows a purple red when the negative glows with a bluish violet. The decomposition of water, which consists of oxygen and hydrogen in the formula H₂O, is readily accomplished by the secondary current, and the greatest volume of gas (hydrogen) will be evolved at the negative pole. For ready reference a summary of these facts is given below:

Although it is customary to use bundles of fine, soft iron wire for coil cores, very excellent results have been obtained with cores made up of soft iron filings. These filings should be tightly packed in the core tube and have a soft iron head at the contact breaker end. Filings demagnetize very quickly and prevent the formation of destructive eddy currents, which have been previously discussed (Chapter I.).

Modern practice tends towards a lengthening of the core and primary, in some cases fully 20 per cent of the core length projects from each end of the coil. One result must be as in electromagnets, the longer the core, the longer it takes to magnetize or demagnetize. But even here it is a matter of individual construction.

The common practice is to make coils to be in a horizontal position; there is no reason why they cannot be made to stand on end. In fact, this position to an extent takes off some of the strain on the primary. It is mostly a matter of choice or convenience.

As to the possible output of an induction coil, it depends upon design and construction; but S. P. Thompson gives the following law in his work on Electricity and Magnetism: The electromotive force generated in the secondary circuit is to that employed in the primary nearly in the same proportion as the relative turns of the two coils.[1]

[1] We do not attempt to reconcile this quotation with the enormous estimates of spark potential.

In selecting a Ruhmkorff coil, it must be remembered that the rating in spark length is subject to question. Supposing two similar coils be operated, one with a rapid vibrator and the other with a slow vibrator, other things being equal, the slow vibrator will give the greatest spark length. Again, the appearance of the spark is of vast importance. Although two coils might be sparking across the same length air-gap, the one giving the whitest and thickest continuous succession of sparks is the better. Fig. 13 shows a reproduction from a photograph of a spark 32 inches long, generated by the coil shown on the frontispiece.

Fig. 13.

It is easy to take a coil, and by snapping the vibrator contacts together a few times a spark of thin bluish character will jump across a gap, of length far exceeding the spark gap when vibrator is working at normal speed. But this spark only passes at irregular intervals, seemingly gathering strength for its forced leap. It must not be considered in rating the coil.

In winding primary coils it is proposed to reduce the self-induction or inductance of its adjacent coils by means of similar methods used in winding electromagnets. The primary winding, instead of being composed of a number of turns of one large wire, is made up of a multiple winding of small wires, aggregating the conductivity of the large wire. This materially reduces sparking at the contact breaker, and certainly allows of a closer bedding of wire nearer the core, also giving a greater percentage of ampere turns. Another scheme which uses the Dessauer contact breaker provides two separate primary windings, opening one when the other closes. Such schemes as these come well within the scope of the experimenter, and it is highly possible that valuable improvements will be made in coil design during the coming years.

Failure to Work.

The following are the commonest causes of coils not working to their best limit: Contact breaker contacts dirty, burned, stuck, too small, not in good parallel relation face to face of platinum.

Secondary wires crossed outside coil, often happens that the secondary is quietly sparking away into or through some object touching it, particularly when long wire connections are run from secondary to place of desired sparking.

Condenser too small, burned out, badly insulated (see other pages on this subject).

Battery too small—too high internal resistance or wires leading from battery to coil too small—for ordinary coil work, distance of, perhaps, ten feet, use No. 10 to 12 B & S flexible lamp cord or solid wire. Ruhmkorff coils require plenty of current to produce large sparks.

As it is not always convenient to procure paper and foil in set sizes, the area of material needed for condensers is also given. The above table is approximate. It represents data collected from the best modern practice. The gauge above given for copper wire is that of Brown & Sharpe, and is used throughout these pages.

Medical Coils.

The main points of difference between coils for electrotherapeutics and Ruhmkorff coils is that the former are devoid of condensers, are rarely insulated to a high degree, and are arranged for current strength regulation. The modes of regulation are many, briefly the principal are: (a) In coils with independent circuit breakers, sliding both core and primary coil out of the secondary together or independently. (b) Moving a metal tube over or off the primary coil or core or both. Many combinations of these methods are practised. Attempts have been made to regulate battery current by rheostat, but it is not feasible, except in large stationary outfits. Cheap medical coils are wound with bare wire, with layers of thread between adjacent turns, or even only bedding the wire turns in paraffined paper. It is not intended to convey the idea that winding bare wire coils is a makeshift; far from it. This method is being very generally adopted in telephone work. But it requires special and delicate machinery, and is unsuited to amateur work, where slight differences of cost or labor are insignificant. Others for specific purposes consist of a primary coil only. The best and most complete made are so arranged that independent secondary coils of different sized wires can be used with the one primary, being readily slipped on or off as required. There is another scheme of regulation, where the coil is wound in sections and these sections cut in or out by means of a switch, but it is not desirable.

Medical Coil with Tube Regulation.

Fig. 14.

Figure 14 shows a coil with tube mode of regulation. The core C consists of a piece of iron tube, very thin, 4 inches long by ⅜ inch diameter, and filled with soft iron wires. One end of this core is firmly fixed in the left-hand bobbin head. The object of the iron tube is to prevent the sliding tube from catching in the iron wires, otherwise it can be dispensed with. Over this tube is slipped a brass tube T, ending in a handle H at the right-hand end; this must work easily over the core tube. The spool for the primary is now made up by fixing the other bobbin head on a paper or fibre tube and fastening its free end to the left-hand bobbin head, or the spool can be made in the usual way by glueing up two spool ends on a fibre or paper tube and securing the iron core firmly in one end, allowing room, of course, for the brass tube to slide in at the right-hand end. The primary winding is three or four layers of No. 20 B & S gauge cotton-covered magnet wire, the ends being brought out for future connection. Over this is now laid a few layers of paraffined paper, and ten or twelve layers of No. 36 B & S cotton-covered magnet wire is wound on for the secondary coil.

The contact breaker R is in no way different from the simple form described in Chapter II. Its construction can be readily seen from the figure.

A layer of cloth of the kind used in covering electromagnets is laid on over the secondary, and the coil is ready to be attached to the base. The base is seven inches long by three wide, and has little feet at its four corners to elevate it from the table and prevent abrasion of the connections underneath.

Fig. 15.

The connections are as given in Fig. 15. When in operation, the electrode cords being attached to binding posts, Nos. 1 and 2 are in circuit with the secondary coil only. When at Nos. 2 and 3 they receive the induced current or extra current in the primary, caused by the break of the battery circuit (see page 3).

Medical Coil with Interchangeable Secondaries.

This form of coil is the only one for practical medical work, and more space will be given to its construction than to the foregoing, which is suited only for limited use.

Fig. 16.

Fig. 16 shows side elevation of coil on base. The design can be largely varied, also it can be used either for a wall board, a cabinet top, or made to be carried in a case containing battery, electrodes, etc. S is one of the secondary coils, of which at least three should be provided. The dimensions are, of course, the same—namely, four inches long by 3½ inches wide over all. The spool ends are furnished with heel pieces, which slide under the brass track bar T. This accurately centres the coil and prevents it from working loose.

Windings for Secondary.

The following windings for removable or interchangeable secondary coils are those most in use.

Coil No. 1. 4500 feet (.375 pound) No. 36 B & S, approximating 1800 ohms. This may be led out in three divisions by means of switch on coil head. First division, 4500 feet; second division, 3000 feet; third division, 1500 feet.

Coil No. 2. 2400 feet (.6 pound) No. 31 B & S, about 350 ohms, divided into 2400 feet, 1500 feet, and 900 feet.

Coil No. 3. 750 feet (1 pound) No. 22 B & S in one coil, or two divisions of 500 and 750 feet, respectively; approximate resistance of wire, 125 ohms.

Coil No. 4. It may be necessary to obtain currents of extremely high tension, in which case a coil may be prepared of 5000 feet No. 38 B & S, or No. 40 B & S preferably.

The finer the wire, the less current and the most sedative effect; the coarser the wire, the more current with corresponding increased painful action.

The spools, in fact as much of the framework as possible, should be made of hard rubber, to which a fine finish can be given, although mahogany, rosewood, or even stained oak can be used. On each side of the right-hand spool heads a flat brass spring is screwed, making the contact for the secondary wires on brass strips screwed on top of the track rods. These secondary connections can be made by means of flexible cords to binding posts, but the sliding contact is preferable. The primary coil P is firmly held in the left spool head, and consists of a core of No. 22 B W G soft iron wires, insulated and wound with three layers of No. 20 B & S magnet wire. The outside of this coil is neatly enclosed in a hard rubber tube to permit of the secondary coils sliding freely upon it. It is better, however, for the secondary coils not to touch the primary tube. The vibrator, or contact breaker, should be of the adjustable form shown in Fig. 17. The adjusting screw for the contact breaker can be mounted in a brass lug carried by the spool head.

Connections of this coil are substantially the same as those of the first-described medical coil. This apparatus is well worthy of elaboration; it should be fitted with a ribbon vibrator as well as an adjustable speed slow vibrator, a switch controlling either. A great variety of secondary coils can be made, those of coarse wire taking the place of the current from the contact breaker. The vibrators should be operated from an independent battery, although in the last coil described the magnet may be wound with the same size wire as the primary and then be in series with it. The secondary spools can be made of stained hard wood ends fitted on to fibre tube, which latter is easily procurable. Particular attention should always be paid to the spools and heads; if not properly made, they may come apart, and a disastrous unravelling of the wires ensues.

Fig. 17.

Bath Coils.

A coil much used for electric baths has a primary winding only, regulated by the sliding in and out of the iron core, which necessitates the use of an independent vibrator, or else by varying the current strength with a rheostat. The general directions given before will answer in the present case, the only data necessary being the size of wire, which should be about six to ten layers of No. 20 B & S. The coil with movable secondaries here comes into service. Strong currents are needed for bath work, and any variety of winding can be used with this make of coil. There are so many descriptions of bath and small medical coils in the electrical magazines published for amateur workers, that it is hardly necessary here to give more than a mention of the principal ones.

Hints in Caring for Medical Coils.

A few remarks on medical coils and their diseases may not be amiss; often a very little defect, if remedied in time, will prevent costly repairs.

The main care in medical electrical apparatus is the battery (see Chapter X. for descriptions of coil batteries and their operation). Clean, fresh solutions and clean contacts are essential. Keep zincs well amalgamated, remove wires from binding posts, and scrape bright the metal where the wires make connection; see no fluid is splashed on contacts, clean all contact springs periodically. The Edison-Lalande battery is probably the best for medical use, but even this requires occasional attention as to contacts, new zincs, fresh solution, etc.

Poor adjustment at contact breaker, dirty or corroded contacts, loose wires, loose binding posts, corroded binding posts, are often the only trouble in a coil refusing to work.

Flexible cords are fruitful of trouble: the tinsel breaks, and there is no circuit; gets wet and crosses or causes a leak; cord tips get loose and alternately open and close a contact; one minute all is well, next minute no current can be obtained. Another trouble in acid batteries is caused by leaving the zincs in the fluid. It is easy to do it in most cases, although the ingenuity of the leading medical electrical apparatus makers to-day is directed to this point. Cleanliness and careful inspection of all contacts is well repaid; carelessness surely brings its evils.

It is very desirable in medical work to eliminate the noise attendant upon the working of the coil vibrator. This jarring or humming is often in itself a source of irritation to a nervous patient. The sound can be deadened in various ways, for instance, by placing over the vibrator a temporary wood cover, lined with felt, resting upon a soft rubber gasket; or in any other manner that may suggest itself to the operator.

Table Showing Resistances and Feet Per Pound of Copper and German Silver Wires.

CHAPTER II.
CONTACT BREAKERS.

Fig. 18.

The simple form of contact breaker already described is useful up to a certain point, but it has disadvantages. Its rate of vibration is only variable through narrow limits, and it is not suitable for very heavy currents. But as it stands it has done long service, and will be used probably wherever the requirements from it are not exacting. The most desirable form of this simple spring break is shown in Fig. 18. R is the soft iron armature; S, the spring; C, check-nut which holds the adjusting screw A from becoming loose; T, a second adjusting screw used to tighten the spring and so raise its rate of vibration; K is the base to which one wire of the coil is attached; L, base of adjusting device to which battery wire runs at I. Where tightening screw T passes through the pillar of the adjusting screw, the hole therein is bushed with rubber to prevent accidental contact. Both A and T are provided with insulating heads of rubber or ivory. At B are the platinum contacts, which should be fully ⅛ inch in diameter.

One serious defect in the action of the simple spring vibrator (Fig. 19) is the tendency of the spring to vibrate, as it were, sinusoidally. This causes an irregularity in the rate of the vibrations, which affects the discharge of the coil very considerably. By far the better plan is to use a very short thick spring riveted to an arm carrying the armature at its end (Fig. 20). R is the armature, S the piece of spring, and K the point of attachment to the base. The actual width of the portion of the spring which vibrates—the hinge portion, it might be called—should not be over ⅛ inch.

Fig. 19. Fig. 20.

The rate of motion is high; but an erroneous notion has been taken of its performance by many persons in the knowledge of the writer. The rate of vibration is not wholly dependent on the size, or, rather, smallness of its spring; the arm and armature considerably alter this, although they are not pliable, by reason of their mass and the momentum consequent on their mass.

A word here on the size of the armature. It should be somewhat larger than the face of the electro-magnet core, and should be thick—that is, in a circular form—say one half its diameter. Of course this does not apply to the steel lever armature before mentioned. It is impossible to lay down arbitrary rules where the conditions are not determined, but a very small amount of experimenting will demonstrate the correct lines on which to build.

When in action, all rapid rheotomes give out a definite musical note whereby the rate of vibration can be determined. Reference to any work on acoustics will show a table of the number of vibrations necessary to produce any stated musical note. The foregoing style of rheotome forms the basis of very nearly all those which are in use. The shorter and stouter a spring the more rapidly will it vibrate, and vice-versa. Carrying out this rule, we can manufacture an instrument which will give as high as 2500 vibrations per second (Fig. 21).

Fig. 21.

The armature A is a piece of flat hard steel bar ¼ × ½ inch, held rigidly on the metal support S and just clearing the upper surfaces of the magnet cores C. The adjusting screw P should be provided with an arm, B B, whereby the rotation of it can be delicately varied. This screw must also be firmly held or the high speed of the armature will jar it loose. A check-nut on each side of the frame carrying it should be provided in every case. The necessary platinum contact can be hammered into a hole drilled before the armature is hardened. The proper place for this contact is about one fourth of the total length of the armature from its support, although in the simple contact breaker it can be placed at the distance of one third if desired. The reason is that the concussion of the adjusting screw dampens the free vibration, and the amplitude thereof is lessened in addition to the counter vibrations of the screw disturbing the regular vibrationary series.

Owing to the fact that the amplitude of the armature vibration is so small, a very delicate adjustment is necessary. The adjusting screw can be placed nearer the free end, but for the reasons given it is not to be desired. The metal bridge should be a solid casting, and the armature clamped by more than one screw.

The mercury vibrator, which is applied to almost every large coil, is as follows:

A pivoted arm carries on one end a soft iron armature, which is attracted by the coil core. The other end is provided with a platinum point adjustable by a set screw. This platinum point dips into a mercury cup—a glass cup containing mercury, with a thin layer of spirits of turpentine. The object of the spirits of turpentine, which is a non-conductor, is to help choke off the spark which would ensue whenever the platinum point was raised from the mercury.

A form of contact breaker which will admit of great variation of speed, and which is adapted to carry large currents, is the wheel-break, constructed in the following manner:

A brass or copper disk 3 inches or more in diameter and upward of ½ inch thick has its periphery divided by a number of saw cuts, which divisions are often filled in with plugs of hard rubber or fibre. This disk is mounted on a shaft, which latter is either the shaft of an electro-motor, or is provided with a pulley by which it can be rapidly rotated. A strip of spring copper on each side of the disk presses upon the toothed surface, one strip being connected to the coil and the other to the battery or other current source. It will now be seen that when the disk rotates the slits or pieces of hard rubber cause the break in the circuit through the brushes or copper strips, the rapidity of the breaks depending upon the rate of rotation of the disk, and the number of slits in the wheel.

The slits or rubber pieces should be one-half the width of the intervening brass, but must be at least one sixteenth of an inch in width, especially where a high voltage is used in the primary coil.

The shaft of the machine may serve as one point of connection in place of one of the copper brushes; but in this event either a wide journal must be used, or else some conducting substance, as plumbago, replace the lubricating oil in the bearings.

Pole Changing Breaker.

Fig. 22 shows a diagram of a pole changing contact breaker which will allow of rapid alternations of current. It is operated by an electric motor by preference, although any motive power can be applied to it.

Fig. 22.

W a W b are two brass wheels, the peripheries of which are broken by the insertion of insulating blocks I I, shown black in the sketch. S S are the shafts on which the wheels are mounted, the two wheels being necessarily insulated from each other. 1, 2, 3, 4 are four brushes of copper pressing on the rim of the wheel and leading in the current from the battery B. The primary coil is attached to the brass body of the wheel or to the shafts. When the wheel is in the position shown, the coil and battery are on an open circuit; but on the wheel commencing to revolve, the brushes 1 and 2 bear on the brass, and the current flows from the positive pole of the battery to 2 through the wheel W a to the coil P, up through wheel W b and out at 1 back to the battery. The next position of the brushes 1 and 2 will be on the insulations, and 3 and 4 will come into action. Then the positive current will reach W b by means of brush 3, and after traversing the primary coil and wheel W a, emerge at 4 to the battery, thus reversing the current through P as many times as there are sets of segments, which latter can be multiplied according to requirements. The main point to be considered after that of good connections is that the brushes 1 and 3 and 2 and 4 do not at any time touch any part of the brass wheel at the same time, as this would short circuit the battery. This is avoided by making the insulating space longer than the brass surface, and adjusting the brushes as in the sketch, that each pair of them is a fraction further apart than the length of the brass tooth.

Accordingly, a wheel may be constructed with many segments and rotated at a high speed and rapid reversals of current produced, the uses of which are manifold.

As will be described in the notes on the Tesla effects, an electro-magnet, the poles of which are brought near the sparking point of the contact breaker, will help wipe out the spark, and so assist the suddenness of the break.

An extremely successful expedient in operating contact breakers is to employ a high-pressure air blast directed point blank against the contact point. The effect of this air blast when the contact is made is of course null, but on the platinum surfaces becoming separated, the high air pressure produced forms a path of extremely high resistance, and tends to blow off the spark as soon as it is generated. The stream of air should issue from an insulated nozzle of glass or rubber, and should not contain moisture.

Wehnelt Interrupter.

One of the most important inventions in coil work is the electrolytic interrupter of Wehnelt. Briefly, the apparatus consists of a vessel containing a solution of acid, into which dip two electrodes connected in series with the source of power and the primary of the coil. Upon passing a current through the combination the fluid becomes agitated at the electrodes and a rapid make and break of the current ensues (Fig. 23).

Fig. 23.

It requires considerable electromotive force for operation, a minimum of 40 volts being desirable. Its rapidity of action varies up to and at times exceeding 4000 interruptions per second. A Wehnelt interrupter can be made as follows: Procure a glass jar J holding about one quart or a little less, also a cover for same C, a piece of sheet lead L large enough to fit loosely across the jar and yet not touch the bottom, eight inches of one-quarter-inch glass tube M, a few inches of No. 20 platinum wire P, and two ounces of mercury. Heat the end of the glass tube in a gas flame, and bend an inch or less at a right angle; at the same time seal in the platinum wire by means of a blowpipe, so that the tip just projects from the bent end of the tube. This sealing can be accomplished readily by one unused to working glass, but almost any philosophical instrument maker will have it done at small cost. Holes being bored through the cover, the lead plate and the glass tube are fitted in, the platinum point almost touching the lead. Adjustment is, however, easy, as the tube, being turned, will retract or advance the platinum point from or towards the lead electrode. Nearly fill the jar with a solution composed of one part sulphuric acid to eight parts water, and fill up the glass tube with mercury. The connections can then be made by means of a clamp on the lead and a wire dipping into the mercury. Connect the lead plate L to one pole of the battery or source of energy, and the platinum-mercury electrode F to one post of primary. The other side of battery and coil being closed, the apparatus will begin to work. No condenser is needed with this interrupter.

Dessauer Contact Breaker.

This is a modification of the spring hammer-head type, but has a platinum contact on both sides of the spring. It thus obtains double vibrations, but is liable to stick. The elasticity of the spring normally prevents the circuit remaining closed on the forward movement of the hammer head, but this combination requires attention.

Steel Ribbon Interrupter.

For light currents and rapid vibrations, such as are employed in electrotherapy, the steel ribbon interrupter is suitable. It consists of a steel ribbon V one-half inch wide by six or eight inches long and the thickness of a stout visiting-card. Near the end is riveted a platinum contact. One end of the ribbon is held by a brass upright R, to which connection is made to circuit; the other end is riveted to a threaded rod, which passes through a brass pillar, and is held by a thumb-screw and check nut S. Turning the thumb-screw either way tightens or loosens the ribbon and so raises or lowers the rate of vibration (Fig. 24).

Contact Breakers in Vacuo.

Contact breakers in vacuo, as applied to Ruhmkorff coils, are by no means of recent date. Poggendorff made use of such prior to 1859, and noted the diminished sparking at the contact breaker and increased effect in the secondary circuit.

Fig. 24.

Fig. 25.

Mr. D. McFarlan Moore, whose experiments in vacuum tube lighting have proven so interesting, was granted patents upon various forms of contact breakers, in which the chief merit was that the contacts were broken in a vacuum. The sparking was almost eliminated, and the suddenness of the break of contact so accentuated as to materially improve the output of an induction coil. A perusal of his patents, copies of which may be procured through almost any bookseller, will prove profitable to the coil constructor.

Queen Contact Breaker.

The most important advantage of this arrangement is the abrupt break, owing to a collar in the vibrator striking a movable contact while at full speed. Reference to Fig. 25 will show that the movable platinum contact is carried on a small vertical spring behind the vibrator spring, and projects through a collar on the vibrator spring. When the contact is made, the movement of the vibrator is not arrested, but continues at its full amplitude, thus allowing a long "make." The vibrator is kept moving at a constant amplitude by means of the small coil shown in the illustration, which is in shunt with the main circuit. In the old forms there has always been a liability of the platinum contacts sticking (or welding together). In the new form, as the break is made when the vibrator is in the middle of its swing, the sudden blow with the entire momentum of the iron hammer head is always sufficient to break the platinums apart. This form of contact breaker is very efficient on electric-light circuits, and operates with the utmost regularity.

The Queen Contact Breaker for Large Coils.

This is a device where the actual break is made in alcohol between large studs of platinum nearly one-quarter inch in diameter. The bottom contact can be raised or lowered by means of an adjusting screw. The top contact is secured into the bottom end of a rod passing down a guide tube into the alcohol to meet the lower contact. By means of an electric motor and a cam motion, the top contact and plunger are made to work up and down in the alcohol, thus making and breaking the current flow. One of the commendable features of this contact breaker is that the platinum studs are caused to revolve while in operation, thus presenting new faces to each other after each blow. The apparatus is not adapted for rapid action, but for the handling of heavy currents.

Adjustable Contact Breaker for Medical Coils.

An adjustable contact breaker for medical coils is shown in Fig. 26. M M are the magnet coils, A is the armature, carrying a platinum contact, which vibrates against the adjusting screw P. The armature is pivoted at J, but is held at a distance from the magnets by the springs S S. The other end of the armature carries a ball B, which can be slid up and down on the rod and set at any point by a set-screw. When the ball is at the end of the armature rod most remote from the magnets, the vibrations are slowest; when moved towards the magnets, the vibrations become more rapid. Adjustment of the two springs S S at R R enables the contact breaker to operate on varying current strength, and also tends to lessen the jerkiness of gravity contact breakers. A flat spring, however, can be substituted for the spiral springs, in which case the pivot would be dispensed with and the spring riveted, as in the hammer form of vibrator. The illustration shows this arranged for a wall board, but it can readily be adapted for table work.

Fig. 26.

Adjustable Cone Vibrator.

Fig. 27 shows a form of contact breaker much used in portable medical coils for slow speeds. It consists of a cone of iron H, mounted on the vibrator spring, and furnished with adjustable contact spring and screw A. Its amplitude of vibration is limited by the two pins mounted on the disc, between which the cone vibrates. The disc is turned by hand, thus moving the pins, and so varying the travel of the cone H to and from the core C. It does not give good results from the fact that the rhythmical movements are disturbed every time the cone strikes against the pins, also at the contact spring striking the contact screw. As we showed before, a really satisfactory contact breaker should have a spring, which allows of no sinusoidal movement. Where a pivoted armature is governed by a spiral spring, the result is a series of steady, rhythmical shocks, provided the adjustments are satisfactory.

Fig. 27.

Coil Head Contact Breaker.

Fig. 28 shows the details of a contact breaker to be attached to the coil head direct. It is often used on very small coils, which, together with a miniature dry cell, is slipped into a pocket case. An important detail in small coils is to use a contact breaker of sufficient size. Most of them are not large enough to stand ordinary usage, the adjusting screw is not of sufficient diameter and the thread soon strips. There is no reason why the adjusting screw, its platinum tip, and the pillar or lug which holds it should not be solidly built, it would certainly require less adjustment. Either single or double check-nuts can be fitted to the adjustment screws of nearly all the forms of contact breakers described.

Fig. 28.

Contacts.

It is absolutely essential that the diameter of contacts for all contact breakers should be as large as possible and their faces filed truly parallel to enable them to easily carry all the current required. One of the main causes of failure of coil is burning of the platinum point and platinum burr, the current being then materially reduced. Large sparks at point of rupture are often indications that the condenser is not working properly—perhaps has broken down or is not large enough. The contacts will sometimes fuse together; at any rate, the excessive sparking is an evidence of waste as much as in a dynamo generator.

The adjustable method of arranging condensers (see Chapter IV.) is here of great value, but it is easy to attach more condenser sections to the contact screw pillar and vibrator pillar and notice result. In the construction of Ruhmkorff coils it is a good plan to make all connections possible on the coil base, instead of inside the condenser chamber. This is done either by means of rubber-covered wires or neat strips of brass, screwed down on the base from points of connection, and, of course, carefully bent over or well insulated from all other leads which they have to cross.

The best makers of induction coils construct their instruments so that they can be readily taken apart with as little detachment of connections as possible.

CHAPTER III.
INSULATIONS AND CEMENTS.

In selecting an insulating compound for apparatus designed to be under the influence of high tension currents, a glance at some of the peculiarities of such currents will not be out of place. Mineral oil is used in many of the converters employed to transform the high voltage currents on the mains of the alternating electric-light systems to the comparatively low voltage used at the points of consumption. Professor Elihu Thomson, in a series of experiments, noticed some interesting facts in the sparking distances of high potentials in oils.

He found that discharges of low frequencies, as 125 alternations per second, were capable of puncturing mineral oils at one third to one half the thickness of an air layer sufficient to just resist punctures by the same discharge; but with frequencies of 50,000 to 100,000 per second, an oil thickness of one thirtieth to one sixtieth was a sufficient barrier.

At a frequency of 125 per second, a half-inch spark in the air penetrated one third to one fourth inch of oil; but at frequencies of 50,000 to 100,000 per second, a layer of oil one fourth of an inch successfully resisted the passage of a spark which freely passed through 8 inches of air.

The effect of drying an oil improved its insulating qualities. (Tesla uses boiled-out linseed-oil.)

He also noted that pointed electrodes could be brought nearer together under oil than balls without allowing a discharge. Flat plates allowed of still greater sparking distances. Tesla notes that oil through which sparks have passed must be discarded, probably owing to particles of carbon being formed.

Paraffin wax has a higher resistance than oil, providing it has not been heated over 135° C. It will stand alternate heating up to 100° C. and cooling, being of lower resistance when hot than when cold. But a serious permanent deterioration takes place when it has been heated over 100°C.; its color, from the normal pure white, changes to a yellowish tint when its insulation is impaired. Paraffin also undergoes a deterioration when heated for a long time even at 100° C., and should never be used for fine work when it is at all yellow. It is always best to melt it in a hot-water bath, not permitting, however, any steam or moisture to come near it. In this climate (United States) it is not so necessary to mix in any tallow to obviate brittleness, the average temperature of most workshops being sufficiently high to keep it from becoming brittle.

Resin oils do not suffer permanent injury from being heated, as does paraffin, but their insulating properties diminish much more rapidly on becoming even warm, the initial resistance of resin oils being lower than that of paraffin.

Paraffin has a fault—its tendency to absorb a slight degree of moisture. It has been found in telephone and telegraph cables saturated with paraffin that this is a very important cause of their deterioration. In Ruhmkorff coils, however, which are intended for operation in enclosed places free from damp atmospheres, the absorption of moisture would be probably reduced to its minimum.

There is one substance which, were it not for its cost, would be far preferable to paraffin for coil work, and that is beeswax. Its cost, however, is generally five times that of paraffin, even when purchased in quantities. It never becomes brittle enough to be damaged in careful handling, its melting point is low, and it does not absorb moisture. But it must be unquestionably pure and clear.

In foreign practice a variety of resinous mixtures are used to insulate the turns of the wire in Ruhmkorff coils.

Equal parts of resin and beeswax used hot, paraffin, resin and tallow, and shellac and resin are employed.

Shellac—that is, the yellow lac—is much used as a varnish for electrical instruments, being dissolved in alcohol to saturation. For dynamo armatures and similar apparatus the shellac varnish is of great service, and many good compounds of shellac, such as insullac and armalac, have been prepared for ready use. But (excluding beeswax) for our purposes paraffin stands pre-eminently at the head of the list.

In using shellac varnish, in high tension work more particularly, care must be taken that the moisture has entirely evaporated. Although a piece of shellacked apparatus may appear perfectly dry, yet when the current is allowed to flow unlooked-for results may appear—it takes hours in a dry atmosphere for shellac varnish to dry. Baking the apparatus in a warm oven is a necessary expedient whenever feasible, care being taken not to burn or decompose the shellac. The proportions most generally used are 1 ounce shellac to 5 ounces alcohol. Stand the vessel containing the mixture in a warm place, and shake it frequently; filtration improves the varnish somewhat.

A ready and efficient varnish for silk is prepared by mixing 6 ounces of boiled linseed-oil and 2 ounces of rectified spirits of turpentine. For paper, 1 part of Canada balsam and 2 parts of spirits of turpentine dissolved in a warm place and filtered before being used. A good insulating cement for Leyden jars and insulating stands is prepared from sulphur, 100 parts; tallow, 2 parts, and resin, 2 parts, melted together until of the consistence of syrup, and sufficient powdered glass added to make a paste. To be heated when applied, this will resist most acids. The resin and beeswax compound is handy when making experimental mercurial air pumps of glass tubes, as it has a fair tenacity, is not too brittle, and is easily used.