Transcriber's Note

This book was transcribed from scans of the original found at the Internet Archive. Tables are represented as images. The index of the original book has been removed, but the catalog of books from the publisher has been included. There was no book cover image in the scans so I created one.

Frontispiece. Plate I. Electrical Conventions.

WIRELESS TELEGRAPH

CONSTRUCTION FOR

AMATEURS

BY

ALFRED POWELL MORGAN

EDITOR MECHANICAL AND ELECTRICAL DEPARTMENT OF THE "BOYS' MAGAZINE"

AUTHOR OF "WIRELESS TELEGRAPHY AND TELEPHONY"

WITH 167 ILLUSTRATIONS

Third Edition, Revised and Enlarged

WITH A COMPLETE DESCRIPTION OF THE

NEW WIRELESS LAW

NEW YORK:

D. VAN NOSTRAND COMPANY

25 PARK PLACE

1914

Copyright, 1910, 1913, by

D. VAN NOSTRAND COMPANY

Stanhope Press

F. H. GILSON COMPANY

BOSTON. U.S.A.

PREFACE.

In this work, the author has endeavored to present a book embracing practical information for those who may wish to build for private or experimental use a set of wireless instruments which are more than toys but yet not so expensive as the commercial apparatus.

Many books have been published on the subject of wireless telegraphy, but in them the interests of the novice have been rather neglected and in order to build an outfit he has been forced to rely upon a series of disconnected articles published in the amateur periodicals.

It is the object of this book to show the construction of simple, efficient instruments by means of clear drawings, and to give enough elementary theory and practical hints to enable the experimenter to build a size and type in keeping with his needs and resources.

The tiresome "how to make" style has been avoided as far as possible. History and all unimportant details are omitted to give in their place a concise explanation of the parts played by the different instruments and the influence of developing their various factors.

A small lathe and a set of taps and dies are necessary to produce apparatus having a good appearance, but a little ingenuity displayed in adapting screws and parts of old electrical instruments oftentimes at hand will make these tools unnecessary.

Ordinary precaution and plenty of time should be used in the work. It is obvious that if a large coil is to be made, it is well to insure its successful completion by painstaking care and the use of proper materials. Neither is it wise to strain an instrument through becoming impatient and using it before it is properly completed and adjusted.

Wherever possible instructions have been given regarding the adjustment of the apparatus, but it is only by actual practice that the operator will acquaint himself with the most efficient manipulation.

Extracts from articles contributed by the Author to Popular Electricity have been used in the chapters on Spark Gaps, Oscillation Detectors and Telephone Receivers, through the courtesy of the editor, Mr. H. W. Young.

In conclusion, the writer wishes to express his thanks to the United Wireless Telegraph Company for views of their apparatus and to the other firms who have loaned electrotypes and supplied information. To those who have assisted in the preparation of the book, more especially to Mr. Safford Adams, for numerous suggestions and criticisms, the Author desires to express his full acknowledgments.

ALFRED POWELL MORGAN.

UPPER MONTCLAIR, N.J.

June, 1910.

PREFACE TO THE THIRD EDITION.

The success of the previous editions of this book has made a new and third one necessary.

There have not been any startling changes or new discoveries made in the field of wireless telegraphy since the first edition was published, but the art has undergone a number of small changes and improvements which have increased the efficiency and selectivity of the apparatus.

Since then a federal law restricting and controlling wireless telegraphy has been passed. Its effect has been to place wireless telegraphy upon a more certain basis, and to give a recognized standing to the amateur experimenter.

This new law has been included in this edition in the form of an appendix. The amateur will do well to read it carefully. Compliance with its regulations will prove beneficial rather than a hindrance.

A fully illustrated chapter explaining exactly how to comply with the law and how to build the apparatus required has been added. Complete descriptions of several new types of detectors are also included.

All old matter has been thoroughly revised and several illustrations replaced by ones more up-to-date and of direct interest.

ALFRED P. MORGAN.

UPPER MONTCLAIR, N.J.

May, 1913.

CHAPTER I. INTRODUCTORY.

Being desirous of keeping this book as far as possible within the limits prescribed by the title, it is not possible to go deeply into the theory of the propagation of electric waves, but at the same time it is not deemed advisable to plunge suddenly into the construction of wireless apparatus without giving some explanation of the underlying principles.

If the reader desires information upon this subject he is referred to Fleming's "Electric Wave Telegraphy" or the same author's "Elementary Manual of Radio-telegraphy and Radio-telephony."

The explanations given in this chapter do not involve any actual theory of the transmission and reception of electric waves. They are merely intended to show the train of actions which take place and may be observed in a physical sense. With this purpose in view, several references have been made to simple hydraulic apparatus and an analogy drawn to render the explanation clearer.

The Transmission and Reception of Electric Waves.

Wireless telegraphy by means of electromagnetic waves may be divided into four distinct operations, namely:

  1. The generation of electrical oscillations.
  2. The transformation of electrical oscillations into electrical waves.
  3. The transformation of electrical waves into electrical oscillations.
  4. The detection of the electrical oscillations.

The first two operations comprise those taking place at the transmitter, while the last two, which are the converse of the first, are in evidence only when receiving.

Fig 1. Hertz Oscillator and Resonator.

Fig. 1 illustrates the original Hertz oscillator and resonator, which is the simplest form a wireless installation may take. T represents the transmitting apparatus and R the receptor. At the transmitting station a telegraph key is placed in series with a battery and an induction coil. Two large metal plates, t and t', are connected to the opposite sides of the spark gap, which in turn is connected to the secondary of the induction coil. When the key is pressed the electrical circuit is completed and the voltage of the battery is raised sufficiently by the induction coil to charge the metal plates t and t'.

The key serves to break the current into periods corresponding to the dots and dashes of the telegraph code. When the high voltage of the induction coil is impressed upon the plates they become charged, and being of opposite polarity, when at a maximum the energy rushes across the gap and produces a disruptive spark. Each discharge, although appearing like a single spark passing in one direction, is in reality made up of a large number of rapid oscillations or surgings. The first passage of current serves to more than discharge the plates and they become charged in the opposite direction. A reverse discharge then occurs which also oversteps itself, and thus the oscillations go on, but gradually become weaker and weaker until they die completely or are damped out. The heated air of the spark gap becomes a conductor during the passage of the spark, and the oscillations are enabled to surge back and forth at the rate of 15,000 to 1,000,000 per second, although the actual discharge may take only a fraction of a second.

Fig. 2. Hydraulic Oscillator.

The generation of electrical oscillations may perhaps be made more clear by reference to the hydraulic apparatus illustrated in Fig. 2. T and T' are communicating tubes divided by an elastic membrane M. The tubes may be likened to the metal plates t and t' or the arms of the oscillator. The membrane may be likened to the layer of air between the knobs which separates the opposite arms of the oscillator. P is a pump connected to the two tubes T and T', and the broken lines in the apparatus represent water. The pump corresponds to the induction coil in Fig. 1, and the water to the secondary currents of the induction coil. When the pump is set in operation, the water is drawn from the tube T and injected into T'. The pump valves prevent it from flowing back. When the level becomes very high in T', the great pressure distends the membrane in the direction shown by the dotted line until finally it bursts and the water is allowed to flow with a rush into the tube T. But the inertia of the water causes it to rise higher in the tube than its final position of equilibrium, while in returning and endeavoring to seek its level its inertia carries it below this position. Thus the water oscillates back and forth until finally it comes to rest.

Similarly the difference of potential of the oscillator arms is not immediately equalized upon the breaking down of the air gap, and the apparatus becomes the seat of extremely rapid electrical oscillations, as explained above.

All space is supposed to be filled with a highly attenuated, invisible and weightless medium called ether. When the electrical oscillations surge back and forth through the arms of the oscillator, portions of the energy are thrown off from the apparatus and travel in enlarging circles like the ripples on a pond. These consist of lines of dielectric stress or electrostatic flux which pass through the ether and constitute electromagnetic waves.

The receptor or resonator R, Fig. 1, consists of a circle of wire having in it a small spark gap capable of minute adjustment. Two metal plates r and r' are sometimes attached to the opposite sides of the spark gap. When the key is pressed at the transmitting station and waves are sent out through the ether, they strike the resonator and set up therein electrical oscillations which pass across the gap in the shape of sparks.

Fig. 3. "Hydraulic" Transmitter and Receptor.

To make the explanation clearer, let us consider Fig. 3 in which two floats or blocks of wood are represented as resting on the surface of a tank or pool of water. One float, A, is connected by a rope and pulley so that by jerking the rope the float may be made to oscillate and cause little ripples or waves to pass outwards in a gradually enlarging circle. When the waves reach the float, B, they cause it to rise and fall with each wave or to oscillate and reproduce the movements of the float, A. Likewise the oscillations set up by a wireless transmitter are sent out into space to be caught and duplicated at the receiving station. Of course this analogy to the propagation and reception of electric waves is not the same as the true electrical actions, but is merely a graphical, representation.

Fig. 4. Simple Wireless Telegraph Transmitter and Receptor.

The wireless telegraph outfit illustrated in Fig. 1 would not serve for more than short distances of a few feet, and so a somewhat similar but more efficient apparatus is employed in practice. Fig. 4 shows such a system in its simplest form. In this case the secondary or high potential leads of the induction coil are connected, one to an earth and the other to an aerial or antenna composed of a number of bare copper wires insulated and suspended from a mast.

All electrically charged bodies are surrounded by an electrostatic field of force, the nature of which in theory is a state of strain.

The action of an induction coil connected as in Fig. 4 is to charge the upper part of the aerial above the spark gap, say with negative electricity and establish a field of force in its vicinity varying in area from a few feet to several miles. When the charge reaches a certain potential it is sufficient to puncture the layer of air in the gap and a spark takes place, setting up electrical oscillations.

Fig. 5. Electric Waves and Lines of Strain.

Previous to the rupture of the spark gap, lines of electric strain or force stretch from the aerial to the earth on all sides as in the center of Fig. 5. A line of force may be defined as a curve drawn in the electric field so that the direction of the curve is the same as that of the electric intensity at that point.

The aerial and the earth act like the two metal plates in Fig. 1 or like the opposite plates of a condenser. As soon as the air gap is punctured it becomes conductive and the aerial charge rushes down into the earth. With the discharge, the strain in the electrostatic field is released and the aerial charge rushes down into the earth, but in so relaxing produces a new current and builds up a strain around the antenna opposite in direction to the first. This process repeats itself very rapidly and electrical oscillations are thus set up in the antenna. Every oscillation changes the direction of the magnetic flux or dielectric strain and causes the imaginary lines which originally stretched from the aerial to the earth to be displaced and the ends terminating at the aerial to run down it and form semi-loops or inverted "U's" standing with their ends on the earth in a circular ripple around the aerial and moving away from it with the speed of light. In Fig. 5 three oscillations are supposed to have taken place. The shortest distance between two adjacent points at which the electric strain is at a maximum in the same direction and period of time is the wave length emitted by the aerial. The separate standing groups of dielectric strain moving away from the antenna are electromagnetic waves. In the figure, the adjacent groups are separated by half a wave length. These waves are emitted at right angles to the transmitting aerial, whence they pass through the ether to the other station. When they reach the receiving aerial they set up electrical oscillations therein which are too weak to be perceptible in the shape of sparks as in the original Hertz oscillator and resonator because of the great distance separating the stations, so they are made to flow through a detector, which in Fig. 4 is represented as being a crystal of a mineral called silicon. When the high frequency currents strike the silicon, they set up a weak pulsating direct current. This action is due to a peculiar rectifying property of the mineral. The direct current flows through the telephone receiver and produces an audible sound. If the aerial and ground were connected directly to the terminals of the telephone receiver, without the silicon, the oscillations would not pass because of the impeding or choking action of the electro-magnets in the telephone receivers.

Tuning.—It is sometimes desirable that messages should be made selective or secretive. It is obvious that if there were several large stations in the same neighborhood they could not all operate at the same time unless some means of preventing the stations from receiving more than one message at a time were possible. This is the object in view of the so-called "tuning" of wireless telegraphy. It also accomplishes a second purpose which is perhaps considered more important than the first. The length of the aerial may be too great or too short for the amount of energy and the length of the waves which it emits or receives. When this is the case, the oscillations are quickly damped out and do not generate very powerful waves or produce strong signals at the receiving station and thus by properly adjusting the circuit all undesirable messages may be cut out as well as the signaling range greatly increased. Every electrical circuit has a definite period or electrical length, determined by its inductance and capacity. A circuit emits waves of only one length for given values of inductance and capacity, and must also be of a certain length before it will respond to waves sent out by another transmitter. The careful adjustment of a circuit to emit or receive a given wave constitutes tuning.

Fig. 6. Resonance Tube.

This may be made more clear by the comparison of an electrical circuit with a column of air. Fig. 6 represents a cross section of a glass tube, T, lying in a horizontal position and containing a cork, C, which can be slid to various positions. By adjusting the cork we are able to obtain various depths of air in the tube from its open end, M, to the cork, C.

When a vibrating tuning fork, F, is held opposite the open mouth and the cork slid back and forth it is found that the sound of the tuning fork is greatly increased in volume at a certain position of the cork. If the cork is then removed from this position the sound decreases in intensity. When the cork is in such a position that the sound of the fork is reenforced, we have secured resonance. When in this condition and the prong of the vibrating fork is moving toward the open mouth of the tube a "condensed" pulse of air travels down the tube and back again, having been reflected at the cork and reaching M just as the prong of the fork begins its excursion away from the open mouth of the tube. When the prong of the fork is moving away from M a "rarefied" pulse of air moves from M to C and back again by the time the prong is ready to begin its next vibration. When the tube is not in resonance, the successive condensations and rarefactions passing up and down the air column interfere with one another and decrease instead of increase the sound of the tuning fork.

If we substitute the sound waves emitted by the tuning fork for high frequency oscillations and the air column for the electrical circuit we may readily see that by adjusting its length, resonance can be produced. If the length of the air column is measured it will be found that the reenforcing of the sound of the fork reaches a maximum when the depth of the air column is one-fourth of the sound wave length given by the fork. Likewise resonance is produced in wireless telegraphy when the length of the circuit is approximately one-fourth the length of the waves. Vice versa, the wave emitted from an ordinary closed circuit transmitter is approximately four times the length of the aerial wire. For example, an aerial 25 meters long will emit waves having a length in the neighborhood of 100 meters.

As stated above, tuning is accomplished and resonance or syntony established by varying the inductance and capacity of the circuit. The capacity of a circuit may be defined as its relative ability to retain an electrical charge, while inductance is the property of an electric circuit by virtue of which lines of force are developed around it.

Capacity and inductance are opposite or reactive in their effects upon a circuit. If the value of one is decreased the influence of the other in increased. Fig. 7 and the following explanation will serve to illustrate this.

Fig. 7. Lag and Lead.

Alternating currents do not always keep step with the voltage impulses of a circuit. If there is inductance in the circuit, the current will lag behind the voltage, and if there is capacity, the impulses of the current will lead. Fig. 7 A illustrates the lag produced by inductance and B the lead produced by capacity. In A the impulses of the current, represented by the full line, occur a little later than those of the volts as represented by the dotted line. In B the effect is just the opposite and the current leads. These reactive effects of inductance and capacity are very pronounced with the high frequency currents of wireless telegraphy, and, as stated before, are the factors which determine the period of the circuit.

Fig. 8. Tuned Hydraulic Transmitter and Receptor.

Tuning is represented graphically in Fig. 8. The two floats A and B are not only resting on the surface of a pool of water as in Fig. 3 but are also suspended from the springs S and S'. The springs will have, like a pendulum, a definite time of rising and falling, or period of oscillation, depending upon their length. If we strike the float A the spring will cause the float to rise and fall at a definite rate and send out a little wave or ripple with every oscillation. If the springs S and S' are of the same length, the float B will be caused to oscillate with every wave sent out by A, for, the periods of the springs being equal, B will be permitted to rise with a wave and fall again just in time to be raised by the next oncoming ripple. On the other hand, if the springs are of different lengths, B may only rise slightly and in falling meet an oncoming wave which will cause it to rise before it has reached its lowest point and so dampen or weaken its oscillations that they either do not become very strong or are entirely obliterated. Thus several floats having different periods of oscillation might be sending out ripples in the same pool, and the float B could be made to respond to any of them by adjusting the length of the spring.

We may also see in this illustration the part that tuning plays in causing the apparatus to emit or receive more powerful impulses. When the rope in the untuned apparatus illustrated in Fig. 3 is jerked, the block A oscillates only once or twice before a new jerk is required to keep it in motion. In Fig. 8 it is quite the contrary, for when an impulse has been given to the float A it will oscillate much longer than the untuned float before it requires to be set in motion again. Likewise the float B in Fig. 8 will oscillate longer and more powerfully than the float B in Fig. 3, when once it has been set in motion.

Fig. 9. Tuned Wireless Telegraph Transmitter and Receptor

Fig. 9 shows a diagram of a simple wireless telegraph system employing an inductance and capacity for tuning the circuits. When the induction coil is in operation it charges a condenser. The condenser discharges through the sending helix and across the spark gap. The sending helix is merely a spiral coil of wire of large diameter, and constitutes the greater part of the inductance in the circuit. Two movable contacts, A and B, make connections with the helix. The spark gap, condenser and lower portion of the helix up to the movable contact A are known as the closed circuit. By shifting A, more or less inductance may be included in the closed circuit until resonance is secured. The aerial, the inductance from the contact B down, the condenser and the ground compose the open circuit. By varying the contact B more or less inductance may be included in the open circuit and its period altered until the oscillatory currents of both circuits flow in the same period of time. The closed and open portions of the transmitting helix form an auto transformer, and the voltages of the open circuit are raised above those of the closed circuit.

The tuned receptor shown in Fig. 9 is the simplest form possible and is known as the single slide system. The tuning coil or helix is much longer in proportion to its diameter than the sending helix, and is made of finer wire, since it does not carry such heavy currents. When the contact is slid up or down on the tuning coil, the inductance of the circuit is varied. Since the oscillating currents in the receiving aerial have the same frequency as those in the radiating aerial, the receptor must have the same relative values of inductance and capacity. This condition is obtained by varying the slider until the signals in the telephone receivers are the loudest.

In practice more than one sliding contact is used, and these together with adjustable condensers make the circuit more complicated. These devices are necessary because oscillations may be forced on a receptor by a near-by transmitter unless other precautions than the "single slider" are taken. Such circuits are illustrated in Plates IV and V. With them it is possible to obtain a considerable degree of selectivity and "tune out" an undesirable message.

CHAPTER II. THE APPARATUS.

It is generally the receiving apparatus which first attracts the attention of the amateur operator, and so it will be considered first here. An efficient receiving set consists of some form of Detector, Tuning Coil, Telephone Receivers, and Condenser.

Other accessories such as adjustable condensers, potentiometer, battery and testing buzzer improve the outfit and make it more complete.

The choice of the type of instruments must be left entirely to the person who is constructing them. His resources will determine whether he is to use 1,000 ohm telephone receivers built especially for wireless work or ordinary ones having a resistance of only 75 ohms. It is therefore best to read carefully the chapters devoted to the different pieces of receiving apparatus and select the type of detector, tuning coil, etc., which it is desirable to use before commencing the construction of any.

For beginners, I would recommend an outfit consisting of a silicon detector, a double slide tuning coil, a condenser of fixed capacity and 75-ohm telephone receivers. Such an outfit with a 50-foot aerial will receive messages about 150 miles. If 1,000 ohm telephone receivers are used, messages may be read up to 400 miles. Much depends upon the location of the station and the ability of the operator.

A more elaborate and efficient set consists of an electrolytic or "Perikon" detector, a transforming tuner, two adjustable condensers, a potentiometer and a pair of 1,000 ohm telephone receivers. This outfit and a 75-foot aerial could be made to receive 500 to 1,000 miles by a careful operator.

Several cuts of wireless apparatus built for private installation are shown both in this chapter and further through the book, to give an idea of how the better instruments of this type are constructed and finished.

Fig. 10. Long-distance Receiving Set.

Fig. 10 illustrates a selective receiving set built by the Long Distance Wireless Company. The set is mounted on a mahogany base and the instruments are finished in polished hard rubber and lacquered brass. They comprise a detector stand so arranged that any of the sensitive minerals used in wireless work may be used. The tuning coil is of the double slide type. The condenser is sealed up in a square lacquered brass tube fitted with hard rubber ends and binding posts. The arrangement and construction of the outfit may be readily understood from the cut.

Fig. 11. Murdock Receiving Set.

Fig. 11 illustrates a receiving set manufactured by the Wm. J. Murdock Company. The tuning coil is fitted with hard rubber composition ends and is wound on a special core which is not affected by temperature changes. The detector is of the crystal type and is of rather unique construction, since the small fixed condenser is mounted in the base of the detector itself.

The Clapp-Eastham set in Fig. 12 employs a receiving transformer which makes great selectivity possible. The detector is mounted at the left-hand side of the outfit. A very sensitive mineral called "Ferron" is used in the detector. The fixed condenser is enclosed in a brass tube fitted with hard rubber ends and is located on the front of the base, directly in the center. A variable condenser of the rotary type is placed at the right-hand corner.

Fig. 12. Clapp-Eastham Receiving Set.

The Prague Electric Company manufacture the apparatus shown in Fig. 13. The cabinet is mahogany and is fitted with a hard rubber cover. A fixed condenser and a double slide tuning coil are mounted within the cabinet. The sliders of the tuning coil project through two long slots in the cabinet. A universal detector mounted on top of the cabinet is so designed that any material may be experimented with or tested.

Fig. 13. Prague Receiving Set.

Fig. 14 shows a receiving set built up from apparatus described in this book. The cabinet is 12 x 16 inches and 4 inches deep. The wood should be 1/2 inch thick, and in order to present a good appearance is preferably of mahogany. As mahogany is sometimes very hard to procure and expensive, some may find red birch an excellent substitute. When stained with a mahogany stain it presents a fine appearance.

Fig. 14. Receiving Set.

Varnishing and polishing are wasted time when applied to the average amateur's instruments in view of the rough handling and scratching which they receive. The best plan is to stain the wood with an oil stain and give it a wax finish. An oil stain contains no varnish but is merely coloring matter and oil. A good coat should be applied with a wide brush and the surplus stain immediately wiped off by rubbing the whole surface with a piece of cheesecloth. As soon as the stain is thoroughly dry the wood is waxed.

Cut up some beeswax into fine shreds and place it in a jar. Pour some turpentine over the beeswax and let the mixture stand for five or six hours, giving it an occasional stir. Allow it to stand further if necessary until the wax melts and then add enough turpentine to give the mixture a consistency similar to that of thick cream. Apply the preparation to the wood with a rag, and then rub with a piece of clean cheesecloth until the finish is hard and dry. Waxing produces a gloss which is not so bright as a French polish but yet is more durable and not so easily scratched or marred.

A double slide tuning coil made as described in Chapter XIV is mounted on top of the cabinet in the rear. A "universal" detector is mounted in the center, directly in front of the tuning coil. A tubular condenser of fixed capacity is placed on the left-hand side of the detector, and a potentiometer on the opposite side. A double point switch placed directly in front of the detector enables the potentiometer to be brought into play when a battery is used. Four binding posts are mounted on the front of the cabinet. The ground and aerial are connected to the left-hand pair and the telephone receivers to those on the right hand. The wiring diagram is shown also in Fig. 14. By placing the switch A on contact 1, the potentiometer is brought into use. When on contact 2, the potentiometer is cut out. The switch B must be opened when the detector is not in use so as not to run down the battery.

Fig. 15. Receiving Set.

Fig. 15 shows a receiving set somewhat similar to that shown in Fig. 14 but more elaborate and efficient. The cabinet in this case measures 14 x 22 inches and is 6 inches deep. A loosely coupled or transforming tuning coil is used in place of the closely coupled double slide type. Two variable condensers are mounted on either side of the tuning coil. The detector and potentiometer occupy the space directly in front of the tuning coil, while a fixed condenser is placed at their right. The two switches for breaking the battery circuit and disconnecting the potentiometer are in front of the detector. The aerial, ground and telephone receiver leads are connected to binding posts mounted on the front of the cabinet.

The batteries, in both cases, are placed inside the cabinet. The details and construction of all the separate instruments will be found in the respective chapters as denoted by the titles.

Transmitting Range.—A simple transmitting outfit capable of sending about two miles consists of the necessary batteries, a one-inch spark induction coil, a small zinc spark gap and a key. The connections of such a transmitting outfit are shown in Fig. 4.

If the same coil is used with a transmitting helix and a condenser, the range may be increased from 3 to 5 miles.

A 1 1/2-inch spark induction coil using a condenser and a transmitting helix will send about 10 miles, and a 3-inch coil under the same conditions about 20 miles. A 4 and a 6 inch coil will transmit about 30 and 40 miles respectively.

Fig. 16. Murdock Transmitting and Receiving Set.

The one quarter kilowatt transformer with a helix and four two-quart leyden jars or an equivalent condenser will transmit at least 50 miles with a suitable aerial. Used as a one half kilowatt transformer with a helix and eight two-quart leyden jars, it will send about 100 miles.

Of course, as in the case of the receiving outfits, these distances are approximate and depend upon the location of the station, the nature of the ground over which the messages are transmitted, the kind of receptor used at the receiving station and the efficiency of the operator himself.

Fig. 16 illustrates a complete Murdock transmitting and receiving set. The transmitting outfit consists of a 15-watt induction coil giving about a one-inch spark, a sending helix, oscillation condenser, a key and a double pole double throw switch for changing the antenna and ground from the transmitting to the receiving instruments or vice versa. The spark gap is mounted on top of the coil. The receiving outfit is the same as that illustrated in Fig. 11.

A complete wireless station—outside of the aerial consists primarily of a source of electrical energy, a transformer or induction coil for charging the oscillation condenser, an oscillation condenser, a transmitting helix, a key for breaking the primary current, a spark gap, an aerial switch, a hot wire ammeter for tuning the transmitting circuits, a detector, a receiving tuner, auxiliary tuning apparatus such as fixed and variable condensers, a potentiometer and battery, and a pair of telephone receivers with a headband.

Other apparatus such as switches, insulators, anchor gaps, testing buzzers, reactance coils, grounding switches, etc., have been described in various places throughout the book and their use suggested whenever it is of any advantage.

The choice of transmitting instruments, as with the receiving apparatus, is left entirely with the experimenter so that he may suit his ideas and means. Wherever possible the range and power of the instruments have been given and suggestions made as to the other apparatus which should be used in connection with them so that the completed outfit will bear some sense of proportion.

Fig. 17. United Wireless Portable Outfit.

Fig. 17 illustrates the portable wireless telegraph set manufactured by the United Wireless Telegraph Company for army service and exploring expeditions or isolated camps. The aerial and the mast can be unloaded, erected, and all parts be ready for operation in fifteen minutes. The mast is made of interchangeable wooden sections. The current for the transmitter is furnished by a portable storage battery. The whole outfit is capable of furnishing efficient service for distances of 25 to 30 miles.

CHAPTER III. AERIALS AND EARTH CONNECTIONS.

The aerial or antenna ordinarily consists of a number of wires elevated in the air to emit or intercept the Hertzian waves. In fitting up a wireless station the location and erection of an aerial are of prime importance, and the successful reception and transmission of wireless messages will depend largely upon its condition.

A few years ago the wireless antenna consisted of a metal plate high in the air and having a wire suspended from it, but to-day usually exists in one of the forms illustrated in Plate II.

The higher an aerial is placed above the surface of the earth, the wider will be its electrostatic field, and consequently more powerful electrical waves will be developed. But after a height of 180-200 feet is attained, the engineering difficulties and the expenses increase so rapidly that few stations exceed it. Other things being equal, the increased range in transmitting varies as the square of the height of the radiating wires. For example, a 25-foot aerial capable of transmitting one mile theoretically will send waves 16 miles if made 100 feet high. The actual ratio is often greater, but much is dependent upon the many meteorological conditions.

After the limit in a vertical direction has been reached, the only remaining possibilities are to increase the surface and spread out horizontally.

The flat top aerials are used on shipboard or wherever it is an advantage to suspend the wires between two masts.

They are especially recommended for amateur use, since they need not be so high as the other aerials, to be efficient. The flat top aerials are directive, that is, they receive or radiate waves better in certain directions. The bent or inverted L type is one of these and exhibits a preference for waves coming from a direction opposite to that in which its free end points. This directive action of an inverted L antenna may be somewhat lessened if the leads are taken off at the center and it is made a T aerial. This is the most common form of flat top aerial in use on ships.

The inverted U type is not used extensively because the two opposite leads or rat-tails make a centrally located operating room necessary. The loop aerial is used by the United Wireless Company, in both their ship and land stations. This type of aerial is well adapted to long waves and close tuning.

The Lodge-Muirhead capacity aerial does not make use of a ground and is rarely seen in this country. Lately the United States Signal Corps have applied it to their balloons where an earth connection would be impossible. The upper part of the balloon is covered with a network of wires which serves as the upper aerial, and a second system of wires is suspended below the balloon to take the place of the ground. By this means they have had little difficulty in establishing successful communication between the balloons and the earth.

The pyramid aerial is the type used by Marconi in long-distance ultra-powerful stations, but is debarred from extensive installation on account of the large cost of erection.

The cage and grid aerials are of the vertical type and are excellent where a high support to elevate them can be secured. They are at present used principally by the Massie and Stone Companies.

Plate II. Aerial Systems.

The desirable feature of an aerial is a quantity known as its electrostatic capacity and is measured by the charge required to raise its potential one unit. An increase in capacity enables more energy to be accumulated in the antenna, and consequently greater radiation results. The capacity of an aerial may be increased by adding wires, but must not be carried too far or the transmitting apparatus will not be able to raise its potential sufficiently. Owing to an effect caused by mutual induction between the wires, the lines of strain are not distributed symmetrically, and the capacity will not vary directly but rather approximately as the square root of the number of wires. In order to decrease this action and use the surface most efficiently, the wires should not be placed nearer than one-fiftieth of their length and preferably farther apart.

The materials used for the insulation and suspension of an aerial must be reliable, so that in event of bad weather the station will not lose energy or be put out of working order because the aerial blew down.

Porcelain cleats or a string of porcelain insulating knobs make inexpensive insulators. The standard insulator for wireless telegraph work is the "Electrose" insulator. These are made of a molded composition, and have iron rings set firmly in the ends so that they can withstand a very heavy strain. Hard rubber is undesirable for an aerial insulator because it becomes carbonized and covered with a conducting layer.

Fig. 18. Electrose Insulators.

High frequency currents permeate copper wire only about one three-hundredth of an inch, and so, in order to increase the surface and decrease the resistance, it is best to make the aerial of stranded wire. A phosphor bronze wire for this purpose which is very flexible but still does not sag or stretch, is composed of 7 strands of No. 20 B. S. gauge. Such a wire 150 feet long suspended vertically and insulated from the earth will have a capacity of from 0.0003 to 0.0004 of a microfarad.

The aerial must receive very particular attention if the station is one kilowatt or over in power. In that case stranded wire is necessary. The insulation of the aerial must be as thorough as possible, and proximity to large conductors such as smokestacks, telephone lines, etc., avoided. Rope stays and guys are advisable in order to prevent dissipation of energy. If wire stays must be used they should be divided up at frequent intervals by insulators.

Fig. 19 illustrates a guy insulator used by the United Wireless Telegraph Company. It is made of two strips of well paraffined wood separated by two porcelain knob insulators.

Fig. 19. Guy Insulator.

Copper wire is the most desirable for an aerial. Iron wire must never be used unless it is very heavily galvanized, and even then it is not to be recommended. Aluminum wire is undesirable except for kite sustained aerials. When used on an aerial and exposed to smoke and other fumes it becomes quickly coated with a layer of oxide. All connections made in aluminum wire must be soldered. This necessity may be better understood when it is explained that electro-magnets on dynamos, etc., are sometimes wound with bare aluminum wire and that the natural coating of oxide on the wire is sufficient insulation to separate the turns.

Fig. 20. Insulating Tube.

Where the aerial enters the building in which the instruments are located it must be very carefully insulated. The simplest method is to bore a hole through the wall and push a porcelain tube through it. The rat-tail or leading-in wire is then passed through and the interstices between it and the tube poured full of melted paraffin.

The best method is to bore a hole in the window pane and pass the wire through a hard rubber insulating tube. Fig. 20 shows such a tube. It is three-quarters of an inch in outside diameter and has an internal bore of three-eighths of an inch. The tube is threaded throughout its entire length. Two hard rubber flanges inch thick and 2 inches in diameter are threaded to screw on the tube. The tube is inserted in the hole in the window pane and the flanges screwed on either side. If a soft rubber washer is placed between the hard rubber flanges and the pane there will be less likelihood of cracking the glass. The leading-in wire is then passed through the tube. The hole in the window pane may be bored by using a copper tube having an external diameter equal to that of the required hole. The tube is set in a brace and used like an ordinary bit, but must be kept well smeared with emery and oil or else it will not cut.

This method of leading in the rat-tail is the only one to be recommended if the transmitter is one-quarter kilowatt or over in power.

Fig. 21. High-tension Cable and Insulator.

The lead-in should be anchored just outside of the window so as to relieve the glass pane and the tube from all strain. Pirelli cable or the high-tension cable which is used for the secondary wiring of an automobile is the best conductor to use for the aerial in the interior of a building. The way to lead it over the ceiling is to support it on a porcelain cleat similar to that shown in Fig. 21.

Many are under the erroneous impression that four times the length of the aerial is the wave length which the station will emit. This is only at the best a very rough approximation, for many undeterminable factors such as the nature and location of surrounding objects, trees, etc., so affect the capacity and inductance of the aerial that the wave length must be determined empirically after the aerial is in operation.

The standard wave length of the United States Navy for ship installations is 425 meters. An inverted L aerial, calculated before erection to have a wave length as near as possible to this, has the following dimensions: Four horizontal stranded phosphor bronze wires (7 strands No. 20 B. S.) each 160 feet long and spaced 5 feet apart, four vertical wires 85 feet long and a 35-foot rat-tail.

It is always desirable that the wave length should be as long as possible, for the waves will then travel farther and are not absorbed to such an extent by trees, etc. The absorption due to trees is said to vary as the fourth power of the frequency.

It is sometimes very convenient to calculate the strain on insulators or masts caused by a horizontal antenna. This is easily found by the following equation:

P equals L² x W/8S

where P is the required strain in lbs., W the weight in lbs. per foot of aerial, L the length of the aerial and S the sag of the wire in feet.

When erecting an aerial, it is best to fasten a pulley at the top of the supporting mast and hoist the aerial up after the pole is in position. Then in case the wires become twisted or broken they may be lowered and repaired without any difficulty.

Erection of an Aerial.—The average amateur aerial is generally from 40 to 60 feet high and supported at one end by a short pole placed on the house and at the other end by a mast set in the ground or lashed to a tree. Fig. 22 illustrates such an arrangement whereby a flat-top T aerial is supported at one end by a short pole fastened to the house and at the other end by a pole set in the ground.

Fig. 22. Flat-top T Aerial.

The flat-top loop aerial is preferred by some amateurs and it is to be recommended for receiving but is an inefficient radiator. When this type of aerial is used the two leading-in wires should be connected to a switch so that when the switch is closed they are connected. The aerial may then be used as a straight-away aerial for transmitting, and by opening the switch, as a loop aerial for receiving. This precaution is advised when a loop aerial is to be used with a low-powered induction coil as a transmitter, for otherwise there will be a loss of energy at the anchor gap.

A large aerial is of no advantage when used with a small transformer or induction coil because it cannot become properly charged.

To erect a flat-top T aerial, first select its location. If possible take advantage of two trees and lash a short pole in the top of each, so that the aerial may be raised up clear of the leaves. Another good plan is to erect a pole at each end of the house. In any case, the distance separating the poles must not be greater than three times the height above ground or the directive action of the aerial will be very pronounced. An aerial 50 to 60 feet high should have a length of from 80 to 100 feet. Stranded wire is no advantage for receiving, but must be used if the transmitter is other than a small induction coil.

Secure two spruce sticks about 2 inches in diameter and 10 feet long. Fasten an insulator 6 inches from each end of the spars and two more each 3 J feet from the ends. This arrangement will separate each of the four wires which compose the aerial by three feet.

The two spars are then laid on the ground at a distance apart equal to the desired length of the aerial. Four wires, either stranded or No. 12 B. S. gauge copper, are cut to equal lengths and fastened to the corresponding insulators. The middle of each wire is found and a long copper wire soldered to it. These four wires constitute the rat-tail or lead-in. They should be of the same length, and are not connected together until they are about to enter the building. A short rope tied to each end of the spars and fastened to the rope which passes over the pulley on the top of the pole, serves as a bridle to prevent the aerial from twisting. The aerial is then hoisted up but allowed to hang slightly slack.

There is considerable difference of opinion among experts as to whether or not the ends of the horizontal wires should be connected, and it is impossible to say with good reason which method is better. However, when erecting a flat top aerial, exercise every care to make all the wires of exactly the same length.

An aerial of the size and type just described will send and receive the following distances.

These distances are only approximate and will vary with the efficiency of the operator and the location of the station.

When any of the transformers described later are used, the aerial should be 80 to 100 feet high. In this case the last named receiving outfit will have a range of from 500 to 1,000 miles.

Ground Connections.—The importance of a good earth or ground connection can hardly be overestimated. Whenever possible commercial stations are located on moist ground or near a body of water so that a good ground may be secured by imbedding zinc or copper plates in the earth or water. A ground on shipboard is easily secured by fastening a conductor to one of the ship's plates.

If the ground connection is poor, the natural period of the oscillation circuit is made irregular and short, so that the currents are choked in passing in and out of the earth. The result is an undesirable rise of potential at the lower end of the aerial and often harmful sparking at the ground connection. The transmitting and receiving ranges of a station are very considerably reduced through a poor earth.

Ground connection can often be obtained in the country by immersing metal plates in a well or a cistern. Where connection is made to a water supply pipe some sort of a ground clamp should be used to insure a good contact.

Fig. 23. Ground Clamp.

An efficient earth for portable outfits may be quickly formed by spreading a large area of wire netting over the ground.

Proper precautions for protection against lightning by grounding the aerial outside of the building should be taken.

The wisest plan is to install a heavy single pole double throw switch outside of the building where the rat-tail enters. The knife of the switch should be connected to the aerial, one contact to the house lead and the other to a heavy wire grounded on the outside of the building as in Fig. 24. When the apparatus is not in use the aerial should be grounded by throwing the switch on the grounded contact.

Fig. 24. Switch for Lightning Protection.

The rulings of the National Board of Fire Underwriters governing this class of work are appended below.

"1. Aerial conductors to be permanently and effectively grounded at all times when the station is not in operation by a conductor not smaller than No. 4 B. S. gauge copper wire, run in a direct line as possible to water pipe on street side of said water pipe within the premises or to some other equally satisfactory earth connection.

"2. Aerial conductors when grounded as above specified must be effectually cut off from all apparatus within the building.

"3. Or the aerial to be permanently connected at all times to earth in the manner specified above, through a short gap lightning arrester; said arrester to have a gap of not over .015 of an inch between brass or copper plates not less than 2 1/2 inches in length, parallel to the gap, and 1 1/2 inches the other way, with a thickness of not less than one-eighth of an inch, mounted on non-combustible, non-absorptive insulating material of such dimensions as to give ample strength. Other approved arresters of equally low resistance and equally substantial construction may be used.

"4. In cases where the aerial is grounded as specified in paragraph 1, the switch employed to join the aerial to the ground connection shall not be smaller than a standard 100-ampere jack-knife switch.

"Notice of wiring done for these installations should be sent to the Board, the same as for all other work."

CHAPTER IV. INDUCTION COILS.

Some means of charging the condenser which produces the oscillatory discharge is necessary. An induction coil is the most practical for the amateur.

The induction coil consists of a primary coil of wire wound around a central iron core and surrounded by a secondary coil consisting of many thousand turns of carefully insulated wire. The primary coil is connected to a source of direct current which also includes an interrupter to "make" and "break" the current in rapid succession. Every "make" of the circuit and consequent magnetization of the core induces a momentary inverse current in the secondary, and every "break" and corresponding demagnetization a momentary direct current. Normally, the induced currents would be equal, but by means of a condenser shunted across the interrupter the circuit when "made" requires considerable time for the current and magnetization of the core to reach a maximum value, while when broken the demagnetization and current drop are nearly instantaneous. The value of the induced electromotive force in a circuit varies as the speed at which the magnetic lines of force cut the circuit, and so the induced e.m.f. at "break" is thus rendered high enough to leap across a gap in the shape of sparks.

The formulas connected with induction coils depend upon conditions which are never met in actual practice and cannot be relied upon. To construct a coil of a given size, it is necessary to use dimensions obtained empirically. Therefore it is well for the amateur to stick closely to lines and hints which are given here or which appear in some up to date book on induction coil building.

Fig. 25. Diagram of an Induction Coil.

For a long time the induction coil was an expensive, inefficient instrument, until wireless telegraphy demanded of it more rigid and efficient design and construction. It was the aim of manufacturers to produce the longest possible spark length with a minimum amount of secondary wire. As a result of this demand, wireless coils are now made with a core of large diameter and give heavier and thicker sparks. The secondary in this case is short and uses wire of large cross section in order to reduce the resistance and minimize the heating.

No one part of an induction coil may be developed to its maximum efficiency without seriously influencing and lowering the efficiency of the other parts. The following suggestions regarding the construction are given that they may prove a useful guide to the amateur coil builder. The parts will be considered in their natural order of construction.

Core.—Some experimenters not quite familiar with the principles of magnetism reason that if an induction coil were provided with a closed core as the transformer, the efficiency of the coil would be materially increased. But this is not so, for the magnetization and demagnetization of the iron cannot take place rapidly enough in a closed core when an interrupted direct current is employed in place of an alternating current.

The core of an induction coil is therefore always straight. For the same reason, it is never solid but is made up of a bundle of soft iron wires in order that rapid changes in magnetism may take place. The wires are always of as high a permeability[1] as possible so as to create a strong magnetic field. Swedish or Russian iron of a good quality is the best, as its hysteresis[2] losses are small. The smaller the diameter of the wire the less will be the eddy current losses and heating, but the permeability is also rendered less and the core will not be so effective, as the amount of iron is thereby decreased and the oxidized surface increased. No. 22 B. S. gauge wire is the best size for the average core.

Wires of a good quality may be purchased already cut to various lengths. To buy them in this form will save a great deal of the labor required in building a core. If the wires are not quite straight they may be straightened by rolling them, one at a time, between two boards. It is best to reanneal the wires in the following manner. Place them in an iron pipe and plug the ends of the pipe with clay. Then lay it in a coal fire until the whole mass attains a red heat. The fire is then allowed to die out gradually with the pipe and wires remaining in the ashes until cool. When cool remove them from the pipe and rub each one with emery paper until bright. After this cleaning, the wires are dipped in hot water and dried. They are then dipped in a good quality of varnish and allowed to dry again.

Fig. 26. Induction Coil Core.

The varnish serves to interpose resistance to the eddy currents generated in the core and renders the losses due to this cause much less. A strong paper tube having an internal diameter equal to the diameter of the finished core is made by rolling the paper on a form and cementing with shellac. When perfectly dry. the tube is removed and the wires tightly packed in it. The following table gives the core dimensions for practical coils of different sizes.

Primary Winding.—The ratio of the number of primary turns of an induction coil to the number of secondary turns bears no relation to the ratio of the primary and the secondary currents. It has been found in practice that two layers of wire wound tightly on the core constitute the best primary. The primary should always be thoroughly shellacked or covered with insulating varnish. Since there is almost no ventilation in the primary the wire must be large enough to avoid all heating. A table containing the various sizes of primary wires is given below.

In large coils, the inductance of the primary causes a "kick back" and sparks are liable to pass between the adjacent turns. For this reason, it is always well to use double cotton covered wire and to further thoroughly insulate it by soaking the primary and core in a pan of melted paraffin and allowing them to harden therein. Afterwards the pan is slightly warmed to loosen the cake of paraffin and the excess of wax removed by scraping with a blunt instrument so as not to injure the wires. Paraffin contracts upon hardening, and the proper method to impregnate a porous substance is to allow it to soak and become set in it upon cooling.

A good method of reducing the "kick back" and also the size of the condenser shunted across the interrupter is to form the primary of a number of turns of smaller wire in parallel, the effect being to give a conductivity equal to a single wire of large diameter and at the same time to make a more compact winding of the primary on the core. This method of winding is very desirable in large coils, as it reduces the cross section of the primary and allows the secondary to be placed nearer the core, where the magnetic field is the strongest.

The primary winding ought to occupy nearly the whole length of the core, since there is no gain in carrying the end of the core very far beyond the end of the primary, for most of the magnetic lines of force bend at the end of the primary and return without passing through the extreme ends of the core.

Insulating Tube.—The successful operation of an induction coil without breaking down when under strain depends largely upon the insulating tube which separates the primary and secondary. Hard rubber tubes are perhaps the best. A tube may be easily built up of several layers of 1/2-inch sheet hard rubber by steaming it so as to soften it and then wrapping it around a form. The tube should fit the primary tightly and be about one inch shorter than the core. After the tube is in place it is poured full of beeswax and rosin in order to fill all interstices and prevent sparks due to the condenser effect of the windings from jumping from the inside of the tube to the primary.

Secondary.—A coil used as a wireless telegraph transmitter must have wire of large cross section in its secondary so as to obtain a heavy disruptive discharge. Numbers 34 and 32 B. S. are generally used for small coils and numbers 30 and 28 B. S. for large coils. Silk covered wire is the usual practice, but enameled wire is coming into use. Cotton covered wire takes up too much space and has poorer insulating qualities.

Enameled wire is insulated by a coating of cellulose acetate, which has a dielectric strength of about twice that of cotton and takes up much less room than silk. There is, then, with enameled wire a great saving in space, and a greater number of turns may be placed on the secondary without increasing its mean distance from the core. The following table shows the comparative diameters of silk and enamel covered wires suitable in size for use on the secondaries of induction coils.

In winding enameled wire it must be taken into consideration that the insulation of enameled wire is rigid and has no give. Consequently, to allow for expansion, enameled wire must be more loosely wound than fiber or silk covered wire. The occasional insertion of a layer of paper in winding will give room for expansion and at the same time not add greatly to the diameter.

Fig. 27. Theoretical and practical form of secondary.

The length of the secondary is generally not much more than one-half the length of the core. Coils giving sparks up to 2 inches in length may be wound in two sections or in layer windings, but the layer winding is not recommended for coils giving sparks over one inch. It is best in a coil of this kind to insert an occasional layer of paper. The paper should be well shellacked or paraffined and be of a good grade of linen. It should project about one-quarter of an inch from the ends of the secondary as shown by the sectional drawing in Fig. 28.

This insertion of paper increases the insulation and renders the liability of sparks jumping from layer to layer much less, as is the case when the layers are very long.

Fig. 28. Layer Winding for Small Coils.

The secondaries of large coils are made up of "pies" or "pancakes" from one-eighth to three-eighths of an inch in thickness. The "pies" are separated from each other by a triple thickness of blotting paper which has been thoroughly dried and then soaked in melted paraffin. In cutting the blotting paper, much labor may be saved if a metal template of the required size is first cut from sheet brass and then laid on the blotting paper, which is cut by scoring around the edge of the template with a sharp knife.

Fig. 29. Section Winder.

The "pies" are wound in a bobbin or form such as is shown in Fig. 29.

The disks or flanges are made of sheet brass and mounted on an arbor so that the form may be placed in a lathe or some other contrivance for revolving it. The core is beveled in order to facilitate the removal of a completed "pie" from the winder. The flanges of the winder are clamped against the core by two nuts placed on either side. The "pie" is removed by unscrewing one of the nuts and removing one of the flanges.

In winding silk covered wire it is first passed through a mixture of beeswax and rosin or a bath of melted paraffin. The excess of wax is removed by passing the wire through a slit made in a pad of paper or by rubbing against a piece of felt. Fig. 30 shows such a contrivance.

Fig. 30. Impregnator for Silk Covered Wire.

The wire passes from the reel over an ordinary spool down into the pan of paraffin, out of the paraffin, over another spool, and rubs against a piece of felt to remove the surplus paraffin. The spools are mounted with a screw and a washer so that they will turn without friction.

The wire is guided, when winding, by the fingers. If it is wrapped with a piece of felt and held between the thumb and forefinger it will run without friction and not cut the fingers. It is necessary that the wire should be closely watched for kinks, etc. which would cause the wire to break. Oftentimes the wire is broken but is held together by the insulation. Therefore each "pie" should be tested for continuity when completed. This is best accomplished by means of a galvanometer and battery. All imperfect "pies" should be rejected, as one of them would cause serious trouble if embodied in the coil. In soldering the secondary wires, acid must not be used as it soon corrodes the fine wires. Rosin is the best flux for this purpose. When building a small coil with a "layer" winding it is absolutely necessary that the wire should be wound on in smooth even layers. In a built-up secondary having "pies" not greater than 1/4 inch in thickness such great care is not necessary.

Fig. 31. Methods of Connecting the Secondary Sections.

Fig. 31 shows the methods of connecting up the pies or pancakes. In A, the inside of one section is connected to the outside of the next, and so on. The maximum voltage which can exist between the adjacent sections in this case is equal to the e.m.f. generated by one "pie" and is equal throughout. In B, the coils are connected alternately inside and out. The voltage ranges from zero at the points where they are connected, to a value equal to twice the e.m.f. developed by one section. It would seem that there would be a saving in insulation space of one-half in the first case, but it is not so since the connecting wire passes between the "pies" and therefore the insulation must be twice as thick or exactly equal to that in the second case. The latter method (Fig. 31 B) is the best and most convenient. When the "pies" are connected in this manner the current must flow through alternate sections in opposite directions. To accomplish this it is not necessary to wind every alternate coil in an opposite direction, but merely to turn them around and connect them with the direction of their windings reversed as shown by the arrows and the bevels in Fig. 31. The connections between the sections must be very carefully soldered.

After the secondary is assembled the coil should be placed in a tight receptacle or tank containing melted paraffin. The tank is then connected to an air pump or aspirator and the air exhausted. The diminution of pressure causes any air bubbles in the windings to expand and be pumped out. After standing a while, the pressure of the atmosphere is readmitted and the place of the bubbles will be occupied by paraffin which has been forced in under pressure.

Mounting.—A coil for wireless work is best mounted as shown in Fig. 32 and used with an independent interrupter. The coil may then be placed under the operating table or on the wall, out of harm's way, and the interrupter on the table, where it is handy to the adjustment of the operator.

Fig. 32. Coil Case.

The case is simply a rectangular hardwood box large enough to contain the completed coil. Two binding posts mounted on the side of the box connect with the primary winding and two on the top of the box lead to the secondary terminals. The box is filled with boiled oil or melted paraffin and sealed up by screwing on the lid. If desirable, the secondary binding posts may be mounted on the top of a short piece of hard rubber rod as illustrated in the drawing.

[1] Magnetic permeability is the conducting power for lines of magnetic force.
[2] See hysteresis under Magnetic Detector.

CHAPTER V. INTERRUPTERS.

We now come to what is the greatest source of trouble and annoyance in an induction coil, namely the interrupter. Too much importance cannot be attached to this instrument, for upon it depends largely the satisfactory working of the coil. The operation of an induction coil and the part played by the interrupter were fully explained in the chapter on induction coils.

An adjustable interrupter is necessary for large coils, that is, one not only whose speed may be governed, but also the time and duration of the break.

Fig. 33. Simple Interrupter.

The rapidity of oscillation of a mechanical interrupter is a very different thing from the speed of break. The ideal speed of break is instantaneous. In wireless telegraphy, very faint signals are heard more distinctly in telephone receivers if the rate of interruption at the station sending them is high. The human ear is somewhat more sensitive to sounds higher than those ordinarily produced in the telephone receivers of a wireless receptor. This seems to argue the use of a high-speed interrupter to make and break the current. But the effect on the coil must also be considered.

In the first place, where a condenser is shunted across the terminals of the secondary as is the case with a wireless transmitter, a high-speed interrupter would be very likely to set up harmful oscillations in the secondary of the coil itself.

Second, if too fast, the rise and fall of the secondary currents will be caused to run into each other, since the break will occur before the primary current has reached a maximum and the reverse secondary current has died away.

Third, the diameter of the core of a wireless coil is generally much larger than that of the ordinary coil, and if a very rapid interrupter is employed there is not time enough to properly magnetize the core before the current is broken.

Fourth, the strength of the losses in the core caused by the eddy currents and hysteresis are proportional to the interruptions in the primary circuit and therefore a low speed will be the most efficient. A rapid interrupter requires a higher voltage and amperage than the same interrupter run at a lower speed.

These are some of the reasons why it is very desirable to use an atomic interrupter or one so adjustable that the rate of the time and duration of the "make" and "break" may be closely regulated. An ideal interrupter is designed to give the longest time possible after contact is established and before the "break" occurs.

It does not pay to construct an interrupter for an induction coil giving sparks up to 2 inches in length. The type of interrupter in use on automobile coils is perfectly well adapted to small coils, and may be purchased complete with the platinum points for as low a price as $1.50.

The mechanical break described below is designed so that various adjustments are possible and it may be adapted to almost any coil. Since it is independent, it need not be mounted directly on the coil, but may be placed in the position most convenient to the operator for adjustment. The interrupter will not operate coils well on an electromotive force above 30 volts, for the excessive voltage causes a spark at the contacts when the circuit is broken and prolongs the decadence of the primary current.

Fig. 34. Independent Interrupter.

Independent Atomic Interrupter.—Fig. 34 illustrates two views of the interrupter. Current is furnished to the electromagnets by a six volt battery independent of the source supplying the coil. The interrupter is set in operation by closing the circuit breaker on the aerial switch. When the primary circuit of the transmitter is then completed by pressing the key, the coil will respond immediately because the interrupter is already in vibration.

The electromagnets (Fig. 35) are a pair of four ohm telegraph sounder magnets. A hole is bored in the center of the top of each magnet core and threaded with an 8-32 tap so that the pole pieces may be fastened thereto, The shape and dimensions of these projections, which must be made of soft iron, are illustrated in Fig. 35.

Fig. 35. Details of Magnets.

A soft iron yoke Y, 2 1/2 x 7/8 x 1/4 inches, connects the bottom of the magnets and supports them in an upright position. An 8-32 machine screw passing upward through the base and yoke holds them firmly. The base is preferably of hard rubber 4 x 3 1/4 x 3/4 inches.

Fig. 36. Details of Moving Parts.

The moving parts are illustrated in Fig. 36. The main spring, D, is a strip of spring steel, 2 1/8 inches long, 1/2 inch wide, and 1/32 inch thick. The soft iron armature, A, is fastened to the spring by means of two small 4-36 machine screws. M is a piece of brass rod, 1 1/2 inches long, bent in the form of a hook and threaded with a 4-36 die to screw in a similarly threaded hole in the back of the armature A. The hooked portion of M is fitted with a small piece of hard rubber rod, R, to insulate it where it comes into contact with the spring, G. The spring, D, carries a second hook, E, riveted to the center of the spring 1 5/8 inches from the lower end. The hook, which is about 3/8 inch long, passes through a hole in the top of the spring, F, and engages it so that it is set in operation by the vibratory motion of the spring, D. The spring, F, is 1 3/4 inches long, 5/16 inch wide, and ir 1/64 inch thick. It carries a platinum rivet 3/4 inch from its lower end. The spring, G, is 2 1/2 inches long, 5/16 inch wide and 1/64 inch thick. A heavy platinum rivet is fastened 2 1/4 inches from the lower end. An elongated hole, 1/4 inch long and 3/16 inch wide, permits the hook, M, to pass through the opening. A 5/32 inch hole, 1 1/4 inches from the bottom, allows the adjusting screw to pass through and make contact with the platinum rivet on the spring, F.

Two rectangular pieces of brass, O, 1 1/4 x 1/2 x 5/16 inches are fastened to the base to support the springs.

Fig. 37. Details of Standard and Screws.

The standard, U, supporting the adjusting thumbscrews is a piece of 3/8-inch brass, 2 1/2 inches high. It tapers from 1 1/2 inches at the bottom to 3/4 inch at the top. A hole 2 1/4 inches from the bottom is threaded with a 10-32 tap to receive the thumbscrew, B. A second hole 1 1/4 inches from the base is threaded with an 8-32 tap to fit the adjusting screw, S.

Both of the adjusting thumbscrews carry heavy platinum points. The standard is held upright to the base by means of two machine screws passing through the base.

A 3/32 inch brass rod 1 3/4 inches long is threaded to fit a hole in the top of the armature. A sliding weight, W, may be clamped in any position on the rod by means of a thumb-screw. Raising or lowering the position of the weight decreases or increases the natural period of vibration of the interrupter. Screwing the hook, M, in or out so as to shorten or lengthen it, decreases or increases the ratio of the make to the break.

Fig. 38. Diagram of Connections for an Independent Interrupter.

Fig. 38 shows a diagram of the connections of the interrupter. The standard, the thumbscrew, B, and the spring, G, form part of the primary circuit of the induction coil. The standard, the thumbscrew, S, the spring, F, and the electromagnets are placed in series with a six-volt battery and connected to the circuit breaker on the aerial switch, so that when the switch is thrown in position for transmitting, the interrupter will be set in operation.

A condenser must be shunted across the larger contacts of the interrupter in order to hasten the demagnetization of the core of the induction coil and create a higher e.m.f. in the secondary. The condenser must be suited in size to the induction coil with which the interrupter is to be used and so the following table is appended to serve as a guide.

The condensers are built up of alternate sheets of tin foil and paraffined paper. Connections are made to the sheets by means of tin foil strips which project out alternately from opposite sides as in the illustration.

The paper should be about two inches larger each way so as to leave a one inch margin on all sides of the tin foil.

Fig. 39. Construction of a Paper Condenser.