Wireless Telegraphy and Telephony Simply Explained
Transcriber's Note
This book was transcribed from scans of the original found at Google Books. I have not transcribed the original book's index. Variant spelling are not corrected. Some illustrations are rotated.
WIRELESS TELEGRAPHY
AND TELEPHONY
SIMPLY EXPLAINED,
A PRACTICAL TREATISE
Embracing Complete and Detailed Explanations of
the Theory and Practice of Modern Radio
Apparatus and its Present Day Applications,
together with a chapter on the
Possibilities of its Future Development
BY ALFRED P. MORGAN
EDITOR MECHANICAL AND ELECTRICAL DEPARTMENT OF THE "BOY'S MAGAZINE,"
AUTHOR OF
WIRELESS TELEGRAPH CONSTRUCTION FOR AMATEURS, ETC.
VERY FULLY ILLUSTRATED
NEW YORK
THE NORMAN W. HENLEY PUBLISHING CO.
132 NASSAU STREET
1916
COPYRIGHT 1915 AND 1912 BY
THE NORMAN W. HENLEY PUBLISHING COMPANY
Composition, Electrotyping and Printing
By J. J. LITTLE & Ives Co., New York
PREFACE
Probably no marvel of modern science so grips the imagination as the mystery of those quivering impulses which go forth invisibly to link a ship sailing over the seas with the shores of the distant land.
The author has endeavored to furnish a comprehensive explanation, in simple language, of the theory and practice of this wonderful art, and to explain, as far as possible, the importance of the position occupied by wireless telegraphy to-day and the possibilities of to-morrow.
The title of this book naturally limits the amount of discussion that can be undertaken, and so, in the space at command, there has not been any real attempt made to enter into any engineering or constructive details further than is necessary to make the text clear.
Much that might properly be made a part of the preface has been embodied in the book, in order to avoid repetition, and to also bring certain matter to the attention of those readers who consider a preface to be merely an opportunity for the author of a book to express opinions very often quite foreign to the title, and so unconcernedly skip it with hardly more than a passing glance.
The author wishes to extend his sincere thanks to Mr. H. W. Young, Editor of Popular Electricity; to Mr. John Firth, to Colonel George P. Scriven, and to the Scientific American, for their kindness in supplying photographs for some of the illustrations, and to his friend, Mr. Safford Adams, who has kindly read the proofs and made many valuable suggestions.
ALFRED P. MORGAN. May, 1915.
TO
NIKOLA TESLA
WHOSE GENIUS HAS HARNESSED ELECTRICITY TO THE DAILY WORK OF MAN AND WHOSE INVENTIONS ARE THE BASIS OF ALL MODERN WIRELESS TRANSMISSION, THIS BOOK IS DEDICATED.
Contents
- [PREFACE]
- [CHAPTER I. INTRODUCTORY: WIRELESS TRANSMISSION AND RECEPTION. THE ETHER. ELECTRICAL OSCILLATIONS. ELECTROMAGNETIC WAVES.]
- [CHAPTER II. THE MEANS FOR RADIATING AND INTERCEPTING ELECTRIC WAVES. AERIAL SYSTEMS. EARTH CONNECTION.]
- [CHAPTER III. THE TRANSMITTING APPARATUS.]
- [CHAPTER IV. THE RECEIVING APPARATUS.]
- [CHAPTER V. TUNING AND COUPLING, DIRECTIVE WAVE TELEGRAPHY.]
- [CHAPTER VI. THE DIGNITY OF WIRELESS. ITS APPLICATIONS AND SERVICE. WIRELESS IN THE ARMY AND NAVY. WIRELESS ON AN AEROPLANE. HOW A MESSAGE IS SENT AND RECEIVED.]
- [CHAPTER VII. THE EAR. HOW WE HEAR. SOUND AND SOUND WAVES. THE VOCAL CHORDS. THE STRUCTURE OF SPEECH.]
- [CHAPTER VIII. THE TELEPHONE TRANSMITTER AND RECEIVER. THE PHOTO PHONE. THE THERMOPHONE. THE SELENIUM CELL. THE SPEAKING ARC.]
- [CHAPTER IX. THE WIRELESS TELEPHONE.]
- [CHAPTER X. REMARKS. THEORY. ACCOMPLISHMENTS. MAXWELL'S HERTZ'S DISCOVERY. THE FUTURE.]
- [CATALOGUE OF GOOD, PRACTICAL BOOKS]
LIST OF ILLUSTRATIONS
[Fig. 3.—A static machine, connected to a Leyden jar.]
[Fig. 4.—A Leyden jar discharging through a coil of wire.]
[Fig. 5.—Curved line representing an oscillatory discharge of a Leyden jar.]
[Fig. 6.—Navy type of Leyden jars.]
[Fig. 7.—The simplest practical transmitter.]
[Fig. 8.—A cross-section of the aerial and atmosphere.]
[Fig. 9.—Under the same conditions, but viewed from above.]
[Fig. 10.—A simple receiving arrangement.]
[Fig. 11.—An amateur aerial and station.]
[Fig. 12.—The Army wireless station at Fort Gibbons.]
[Fig. 13.—Lightning discharge near Montclair, N. J.]
[Fig. 15.—Vertical aerials of the grid, fan and inverted pyramid types.]
[Fig. 16.—A diagram showing pyramid aerial.]
[Fig. 17.–A diagram illustrating the directive action of a flat-top aerial.]
[Fig. 18.—Aerials of the "V" and inverted "L" types.]
[Fig. 19.—A diagram showing the arrangement of a "T" aerial.]
[Fig. 20.—Flat top aerials of the inverted "U" and "T" types.]
[Fig. 22.—An amateur aerial (flat top).]
[Fig. 23.—Diagram showing the difference between loop and straightaway aerials.]
[Fig. 24.—Showing how wires are arranged and insulated.]
[Fig. 26.—Leading-in insulator.]
[Fig. 27.–A side view of the aerial shown in Fig. 22.]
[Fig. 28.—Diagram showing how batteries may be arranged.]
[Fig. 29.—The power plant of a Marconi transatlantic station.]
[Fig. 31.—Magnetic phantom formed by a bar magnet.]
[Fig. 32.—Magnetic phantom formed by a wire carrying current.]
[Fig. 33.—Magnetic phantom formed by a coil of wire carrying current.]
[Fig. 34.—Diagram of induction coil.]
[Fig. 35.—Induction coil for wireless telegraph purposes.]
[Fig. 36.—Induction coil primary and secondary.]
[Fig. 37.—Interrupter for induction coil.]
[Fig. 38.—Electrolytic interrupter.]
[Fig. 39.—Open and closed core transformers.]
[Fig. 40.—Lines representing direct and intermittent direct currents.]
[Fig. 41.—Diagram representing alternating current.]
[Fig. 42.—High potential humming transformer.]
[Fig. 43.—High potential closed core transformer for wireless work.]
[Fig. 44.—Leyden jar set for oil immersion.]
[Fig. 45.—Oil immersed condenser.]
[Fig. 46.—Diagram showing construction of condenser.]
[Fig. 49.—Close coupled helix.]
[Fig. 51.—Circuit showing tuned transmitting system employing close coupled helix.]
[Fig. 52.—Photo of spark gap.]
[Fig. 53.—Quenched spark gap.]
[Fig. 54.—Diagram of aerial switch.]
[Fig. 55.—Photo of aerial switch.]
[Fig. 58.—Photo of wireless key.]
[Fig. 59.—Key and aerial switch.]
[Fig. 60.—Portable receiving set and case.]
[Fig. 61.—Complete receiving outfit.]
[Fig. 63.—Complete receiving set.]
[Fig. 64.—Showing the construction of a watch case telephone receiver.]
[Fig. 65.—Pickard adjustable telephone receivers.]
[Fig. 66.—Illustrating the valve action of a rectifying detector.]
[Fig. 67.—A new type of silicon detector.]
[Fig. 68.—Diagram drawing analogy between rectifying action of a detector and pump.]
[Fig. 72.—Electrolytic detector.]
[Fig. 73.—Electrolytic detector in circuit.]
[Fig. 75.—Diagram showing how potentiometer is connected in a circuit.]
[Fig. 76.—Analogy between swinging and tuning.]
[Fig. 77.—Receiving a message in a Marconi transatlantic station.]
[Fig. 78.—Tuning coil of the double slide type.]
[Fig. 79.—Diagram showing fixed condenser in circuit.]
[Fig. 81.—Rotary variable condenser.]
[Fig. 82.—Interior of rotary variable condenser, showing construction.]
[Fig. 83.—Dr. Seibt's rotary variable condenser.]
[Fig. 84.—Sliding plate variable condenser.]
[Fig. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.]
[Fig. 86.—Chain and ball arranged to illustrate the effect of tuning.]
[Fig. 87.—Loose coupled helix.]
[Fig. 89.—The principle of the hot-wire ammeter.]
[Fig. 90.—Diagram showing loose coupled helix in circuit.]
[Fig. 91.—Loose coupled tuning coil.]
[Fig. 92.—Loose coupled tuner.]
[Fig. 93.—Diagram showing position of loose coupler in circuit.]
[Fig. 94.–Fort Gibbons, Alaska, wireless station.]
[Fig. 95.—Transmitting condenser.]
[Fig. 96.—Braun's method for directing wireless telegraph signals.]
[Fig. 97.—Bellini-Tosi radio-goniometer.]
[Fig. 98.—Arrangement of Bellini and Tosi for directive wireless telegraphy.]
[Fig. 99.—Complete receiving and transmitting outfit.]
[Fig. 100.—Special lightweight wireless telegraph set for airship service.]
[Fig. 101.—Telefunken wireless cart, showing transmitter.]
[Fig. 102.—Telefunken wireless cart for military service.]
[Fig. 103.—Telefunken wireless wagon set in operation at Fort Leavenworth.]
[Fig. 104.—Wireless room aboard the U. S. transport "Buford".]
[Fig. 105. The apparatus set up for operation.]
[Fig. 106.—Wireless equipped automobile.]
[Fig. 107.—Co. Signal Corps at San Antonio.]
[Fig. 108.—U. S. Signal Corps pack set shown open and closed.]
[Fig. 109.—The receiving apparatus of the airship "America".]
[Fig. 110.—Interior of the N. Y. Herald Press station.]
[Fig. 111.—Operating the U. S. Signal Corps airship wireless apparatus.]
[Fig. 112.—The N. Y. Herald station, showing aerial.]
[Fig. 116.—Transmitting equipment of the high-power station at Nauen.]
[Fig. 117.—Duplex receiving apparatus.]
[Fig. 118.—Breaking-in system.]
[Fig. 119.—The receiving apparatus of the station at Nauen.]
[Fig. 120.—Diagram of the ear.]
[Fig. 123.—Experiment showing sounding bodies are in vibration.]
[Fig. 124.—Method of registering vibrations of a tuning fork.]
[Fig. 126.—Illustrating the action of air waves.]
[Fig. 127.—The vocal chords in position for making a sound.]
[Fig. 128.—The vocal chords when relaxed.]
[Fig. 129.–Koenig's manometric flame apparatus.]
[Fig. 130.—Appearance of manometric flame in revolving mirror.]
[Fig. 131.—Diagram of a telephone transmitter.]
[Fig. 132.—Diagram showing the principle and construction of the telephone receiver.]
[Fig. 134.—Photophone receiving apparatus.]
[Fig. 135.—Photophone transmitting apparatus.]
[Fig. 136.—Powerful searchlight arranged to transmit speech over a beam of light.]
[Fig. 138.—Circuit showing how a singing arc is arranged.]
[Fig. 139.—A logical form of wireless telephone which is impractical.]
[Fig. 140.—DeForest wireless telephone equipment.]
[Fig. 141.—Wireless telephone receiving apparatus (induction method).]
[Fig. 142.—Fessenden wireless telephone transmitting phonograph music.]
[Fig. 143.—Diagram illustrating why damped oscillations will not carry the voice.]
[Fig. 144.—How the sound waves of the voice are impressed upon undamped oscillations.]
[Fig. 145.—Arrangement of the speaking arc.]
[Fig. 146.—Diagram showing how a wireless telephone transmitting system is arranged.]
[Fig. 147.—Poulsen wireless telephone equipment.]
[Fig. 148.—The Majorana wireless telephone transmitter.]
[Fig. 149.—Showing the brush discharge from a Marconi transatlantic aerial at night.]
[Fig. 150.—An amateur wireless' telegraph station.]
[Fig. 151.—The high-power naval wireless telegraph station under construction at Washington, D. C.]
[Fig. 153.—The aerial system of a transatlantic station.]
[Fig. 154.—Fong Yee, a Chinese amateur wireless operator.]
[Fig. 155.—Tesla world power plant.]
[Fig. 156.—Twenty-five-foot sparks from a Tesla transformer.]
CHAPTER I. INTRODUCTORY: WIRELESS TRANSMISSION AND RECEPTION. THE ETHER. ELECTRICAL OSCILLATIONS. ELECTROMAGNETIC WAVES.
Wireless telegraphy, that marvelous art which has made possible the instantaneous transmission of intelligence between widely distant parts having no apparent physical connection save that of the earth, air, and water, is one of those wonders of science which appeal to the average mind as either incomprehensible or only explainable through the use of highly technical language. Contrary to this general opinion, however, the whole theory and practice of the wireless transmission of messages is capable of the simplest explanation.
FIG. 1.—Throw a stone into a pool of water and little waves will radiate from the spot where the stone struck.
Throw a stone into a pool of water. A disturbance is immediately created, and little waves will radiate from the spot where the stone struck the water, gradually spreading out into enlarging circles until they reach the shores or die away. By throwing several stones in succession with varying intervals between them it would be possible to so arrange a set of signals that they would convey a meaning to one who is initiated, standing on the opposite side of the pool. The little waves are the vehicle which transmits the intelligence, and the water the medium in which the waves travel.
Wireless telegraph instruments are simply a means for creating and detecting waves in a great pool of ether.
Scientists suppose that all space and matter is pervaded with a hypothetical medium of extreme tenuity and elasticity, called luminiferous ether, or simply ether.
Although ether is invisible, odorless, and practically weightless, it is not merely the fantastic creation of speculative philosophers, but is as essential to our existence as the air we breathe and the food we eat. By imagining and accepting its reality, it is possible to explain and understand many scientific puzzles. The universe is a vast pool of ether. It is all-pervading. There is no void. It is diffused even among the molecules of which solid bodies are composed. The study of this substance is, perhaps, one of the most fascinating and important duties of the physicist.
Ninety million miles away from our earth is a huge flaming body of vapors and gases, called the sun. This seething mass of flame and heat furnishes us more than mere winter and summer and night and day, for we on this earth are not living on our own resources, and the real work of the world so necessary for even bare existence is accomplished by the energy of the sun stored up in coal, in plants and trees and mountain torrents.
Light is known to be vibrations of an extremely rapid period—electromagnetic waves, they are called. Heat can be shown to be of the same nature. Traveling at the rate of over 180,000 miles per second, these two great gifts of the sun come streaming continually down to us over the inconceivable distance of almost 100,000,000 miles. Both require a medium for their propagation. The ether supplies it. It is the substance with which the universe is filled. Incidentally it is also the seat of all electrical and magnetic forces.
FIG. 2.—A Leyden jar is a glass jar lined inside and out with tinfoil for about two-thirds of its height.
In throwing the stone into the pool of water, muscular energy of the arm is transferred to the stone, and the latter, upon striking the surface of the pond, imparts a portion of that stored energy to the little waves which are immediately created in the water. In setting up electromagnetic waves for wireless communication the energy imparted to the ether is electrical energy, developed by certain interesting instruments explained further on.
Let us consider briefly how the waves are created in a wireless telegraph station. Almost every one has seen and heard the brilliant snapping spark produced by the discharge of a Leyden jar. A Leyden jar in its common form is a glass jar lined inside and out with tinfoil for about two thirds of its height. A brass rod, terminating in a knob, connects below with the inner coating, usually by means of a loose chain. It may be described as a device which is capable of storing electricity in the form of energy and discharging this energy again in actual electricity.
This discharge has been the subject of many interesting investigations of direct interest.
FIG. 3.—A static machine connected to a Leyden jar.
The inner and outer coatings are connected to the terminals of a static electric machine (an apparatus for generating electricity), and the machine set in rotation. After the jar has been charged, the electric machine is disconnected and one end of a coil of heavy wire connected to the outside coating, while the other end of the wire is made to approach the knob connected with the inner coating. Before the end of the wire reaches the knob a discharge occurs through the coil, producing a noisy brilliant spark between the wire and the knob. The discharge appears like a single spark, but in reality it is composed of a great many following each other in rapid succession. The jar discharges its energy, first by a tremendous rush of current in one direction, and then another discharge somewhat smaller than the first in the opposite direction. There is a series of these discharges in reverse directions, but each discharge is less and less, until the whole amount of energy is expended. The complete series of discharges takes place in an almost immeasurable fraction of time. It is from this phenomenon that the electrical term "high frequency oscillations," so often heard of in "wireless" parlance, is derived.
FIG. 4.—A Leyden jar discharging through a coil of wire produces a brilliant spark and high frequency oscillations are created.
FIG. 5.—Curved line representing an oscillatory discharge of a Leyden jar.
FIG. 6.—Navy type of Leyden jars. Coated with copper deposited upon the surface of the glass.
High frequency oscillations are the "pebbles" which, dropped into the vast pool of ether, everywhere, set up "ripples" called electromagnetic waves (identical with the electromagnetic waves of light, but longer and so beyond the limits of our spectrum and the vision of the eye). The manner in which this is accomplished may be explained by saying that the charge creates a state of strain in the surrounding ether, and then abruptly releases it. Ether possesses a high degree of elasticity, so that when the state of strain is thus suddenly released, it immediately returns to its former state. The sudden motion of the ether results in waves which spread out from their source in enlarging circles.
Wireless telegraphy, as it is practiced to-day, is based upon the fact that a system of wires or circuits, through which high frequency oscillations are surging, becomes a source of electromagnetic waves. Various methods have been devised for making the system more efficient and capable of giving better results with a given amount of power.
FIG. 7.—The simplest practical transmitter that it is possible to devise for the purpose of sending messages.
Fig. 7 is a diagram showing the simplest practical transmitter that it is possible to devise for the purpose of sending messages a sufficient distance to be of any value.
It would be impractical to use a static electric machine for wireless transmission, and so an induction coil or transformer is employed. These latter instruments are for the purpose of raising electric currents of a comparatively low voltage to the high potential, where they have the power of generating high frequency oscillations.
In the illustration the current from a battery is led into the primary of an induction coil. The primary is simply a coil consisting of a few turns of wire, which induces a high voltage in a second coil consisting of a larger number of turns, and called the secondary. The terminals of the secondary are led to a spark gap—an arrangement composed of two polished brass balls, separated by a small air space. One of the balls, in turn, is connected to a metal plate buried in the earth, and the other to a network of wires suspended high in the air and insulated from all surrounding objects.
As noted above, a Leyden jar consists of two metallic coatings, separated by a wall of glass. The purpose of the coatings is to form a conductor and carry an electric charge. A Leyden jar possesses a characteristic called, in electricity, capacity. Any two conductors separated by an insulating medium possess "capacity" and all the properties of a Leyden jar or condenser.
The waves generated by a Leyden jar would be somewhat weak and confined to its own immediate neighborhood, so recourse is had to the aerial and ground, in order to increase the area over which the oscillations exert their influence in setting up the electric waves. The aerial system corresponds to one coating of the Leyden jar, and the ground to the other. The insulating medium in between, corresponding to the glass, or dielectric, is the atmosphere.
When the key connected to the induction coil is pressed, the battery current flows through the primary and induces a high voltage current in the secondary, which charges the aerial and ground exactly as the static machine charges the two coatings of the Leyden jar. A spark then leaps across the spark gap and the current surges back and forth through the aerial, generating "high frequency oscillations" which, in turn, set up a state of strain in the surrounding ether, and cause the waves to travel out from the system.
FIG. 8. If a cross section of the aerial and atmosphere could be made in the same manner that an apple is sliced with a knife and the waves held stationary, they would appear as above.
These waves follow the contour of the earth, and so may cross mountains and valleys, and travel anywhere. They radiate from the aerial like the ripples from a pebble in a pool of water, in gradually enlarging circles. If a cross section of the aerial and atmosphere could be made in the same manner that an apple can be sliced with a knife, and the waves held stationary long enough to see them, they would appear as in Fig. 8. The curved lines represent the lines of strain induced by the oscillations. Each group of lines represents a wave. It will be noticed as they radiate farther from the aerial that they become larger and spread out.
FIG. 9.—Under the same conditions, but when viewed from above, the appearance would be that of a series of concentric circles.
The electromagnetic waves have the power of exciting oscillations in a conductor on which they impinge. This is made use of for the purpose of receiving the messages. When the waves strike the aerial of a distant station they set up high frequency oscillations, which are usually too weak to make their presence known except with the aid of a sensitive device, called a detector.
FIG. 10.—A simple receiving arrangement. The detector rectifies the oscillatory currents passing from the aerial to the ground so that they will flow through the telephone receiver and register as sound.
The most prominent type of detector in use to-day is a crystal of silicon, iron pyrites, zincite or certain other minerals. The mineral is placed between two contact points, one or both of which are adjustable so that the most sensitive portion of the mineral may be selected. A telephone receiver is connected across the terminals of the detector. When the electromagnetic waves from the transmitting station strike the aerial of the receiving station, they set up therein a series of high frequency oscillations, corresponding to the Morse signals emitted from the transmitter. The oscillations flow back and forth through the aerial and ground, striking the mineral detector on their journey. The high frequency oscillations are alternating currents, because they reverse their direction many thousand times per second. Such a current will not pass through the telephone receiver, because the little magnets contained therein exert a choking action on alternating currents of high frequency and effectually block their passage. The mineral detector acts as a valve, allowing the current to pass through in one direction, but not permitting it to return or go in the opposite direction. The result is a series of impulses flowing in one direction only, and therefore called a direct current. Such a current will flow through a telephone receiver and produce a motion of the diaphragm which imparts its motion to the surrounding air, the result being sound waves audible to the ear. By varying the periods during which the key is pressed and the oscillations are being produced, according to a prearranged code, the sounds in the receiver may be made to assume an intelligible meaning.
CHAPTER II. THE MEANS FOR RADIATING AND INTERCEPTING ELECTRIC WAVES. AERIAL SYSTEMS. EARTH CONNECTION.
Every radiotelegraphic station may be summed up as comprising these elements: first of all, certain appliances collectively forming the transmitter and serving to create the waves; secondly, the receiving apparatus, whose function is to detect the signals of some far-distant sending station, and lastly, an external organ called the aerial, or antenna, consisting of a huge system of wires elevated high in the air above all surrounding objects, either vertically or sloping, or partly horizontal and partly vertical, which radiates or intercepts the electromagnetic waves, accordingly as the station is transmitting or receiving.
The antenna is at once both the mouth and the ear of the wireless station. Its site and arrangement will greatly determine the efficiency and range of the apparatus.
The site selected is preferably such that the aerial will not be in the immediate neighborhood of any tall objects, such as trees, smokestacks, telephone wires, etc., because such objects not only absorb an appreciable amount of energy when the station is transmitting messages, but also noticeably shield the aerial from the effects of incoming signals and limit its range.
The nature of the ground over which the waves must travel also enters into the question, and is always considered in locating a station. In gliding over the surface of the earth, the waves generate weak currents in the earth itself. If the ground is very stony or dry, these earth currents encounter considerable resistance, and the possible distance of transmission over soil of this sort is very much less than if it were moist. Moist soil and water offer very little resistance, and the difference in the results obtainable at the receiving station when the waves travel over an area of this sort is very marked.
FIG. 11.—An amateur aerial and station.
A station which can only send 100 miles over land can send messages three or four hundred miles over the ocean.
Forests exert a very decided effect upon the electric waves. Each individual tree acts as an antenna, reaching up into the air and absorbing part of the energy. The difference in the range of a station during the summer months and that of the same station in winter is considerable. In summer the trees are full of sap and, being much better conductors of electricity when in this condition, act in the capacity of innumerable aerials rising in the air, and able to absorb appreciable amounts of energy. During these same months the air becomes highly ionized, in which state the air molecules carry an electric charge, and are particularly opaque to the waves. This condition also usually exists in the presence of sunlight, the result being that the most favorable time for the wireless transmission of messages are the hours around midnight.
FIG. 12.—The Army wireless station at Fort Gibbons, Alaska, showing steel lattice work mast and aerial system.
Locality is another factor which usually receives a fair share of attention in selecting the site. Certain sections of the country, for seemingly no apparent reason, are very hard to transmit messages, either to or from. Wireless stations located on the Pacific Coast, for instance, are more efficient than those situated along the Atlantic seaboard, while those in the tropical regions are only able to send short distances in comparison to those farther north or south. Messages seem to travel better in the direction of the lines of longitude than along the lines of latitude.
FIG. 13.—Lightning discharge near Montclair, N. J.
"Static," that "bugbear" of the wireless operator, is very much more in evidence in the eastern parts of the United States and in South America than it is on the western coast of the country. If any one should ask a wireless operator what "static" is, he would probably reply, "a nuisance." In reality, it is caused by atmospheric electricity. When atmospheric electricity "jumps," it is called "lightning." A lightning discharge sets up very powerful waves in the ether, which strike the aerial of the wireless station and produce a peculiar rumbling, scratching sound in the telephone receivers, and sometimes seriously interfere with a message. In fact, it is possible for a wireless operator to predict a thunder shower by many hours from the sounds he is able to hear in his telephone receivers.
The cause of lightning is the accumulation of electric charges in the clouds. The electricity resides on the surface of the particles of water in the cloud. These charges grow stronger as the particles of water coalesce to form larger drops, because, as they unite, the surface increases proportionally less than the volume and, being forced to lodge on a smaller space, the electricity becomes more "concentrated," so to speak. For this reason the combined charge on the surface of the larger drop is more intense than were the charges on the separate particles, and the "potential" is increased. As the countless multitudes of drops grow larger and larger, in the process of forming rain, the cloud soon becomes heavily charged.
Through the effects of a phenomenon called "induction," a charge of the opposite kind is produced on a neighboring cloud or some object of the earth beneath. These charges continually strive to burst across the intervening air and neutralize each other. As soon as the potential becomes sufficient to break down this layer of air, a lightning stroke from one to ten miles long takes place. The heated air in the path of the lightning expands with great force, but immediately other air rushes in to fill the partial vacuum, thus producing atmospheric waves, which impress the ear as the sound called thunder.
Wireless stations belonging to the United States navy and located on land are usually housed in a small building in the immediate neighborhood of the tall wooden mast which supports the aerial. Commercial stations are usually situated on the top floor of a high office building, or a hotel, and the aerials supported by a steel lattice-work tower. Amateurs place a small pole on the roof of the house, or in a tree, and locate their station in any convenient room near the top of the house.
FIG. 14.— Photo of double lightning discharge passing to earth near the First Orange Mountain, Montclair, N. J.
FIG. 15.—Vertical aerials of the "grid," "fan" and "inverted pyramid" types.
Aerials are of numerous classes and forms, but the most prominent types can be divided into two main groups, called respectively, the "Flat-top" and "vertical" antenna.
The vertical aerials are the older form, and are usually employed for long-distance work or ultra-powerful stations. The aerials intended for transmission from Europe to America, installed by Marconi, consisted of huge inverted pyramids, supported by four heavy lattice-work towers, over 200 feet high. Vertical aerials also sometimes take the form of an umbrella, or fan, where only one supporting pole is available. Iron pipe masts may be employed for the purpose, by setting on an insulating base. The umbrella aerial is used extensively in the army and portable sets.
The flat-top aerials are gradually coming into very extended use. They are used to the exclusion of all others on shipboard. They need not be so high as a vertical type aerial in order to be as efficient. Flat-top aerials consist of a vertical portion and a nearly horizontal portion. The horizontal portion is practically useless, as far as its work in radiating waves is concerned, it being used for the purpose of increasing the capacity of the aerial. An increase in capacity in an aerial means that more energy can be stored and radiated. Flat-top aerials have the objection, however, of possessing a directive action; that is, they receive, or radiate waves, better in one direction than in the other. A flat-top aerial always receives or transmits better in the direction that the ends point than in a direction at right angles to the wires.
FIG. 16.—A diagram showing pyramid aerial.
The accompanying diagram is an illustration to show the effects of the directive action of a flat-top aerial. The black lines marked A B, and appearing very much like a little grating, represent an aerial of the inverted "L" type, looking down on it from above. B is the free end of the aerial, and A the closed end, or end to which the wires leading down to the station are attached. If a snapshot of the lines of strain produced in the ether as the waves move away from the aerial could be taken, they would appear like the curved lines in the illustration. It can be readily seen that those passing outward from the aerial in a direction opposite to that in which the free end points are the strongest, and that the radiation in that direction is the best.
FIG. 17.–A diagram illustrating the directive action of a flat-top aerial.
FIG. 18.—Aerials of the "V" and inverted "L" types.
The "V" aerial and also the inverted "L" type both receive waves much better when they come from a direction opposite to that in which the free end points.
FIG. 19.—A diagram showing the arrangement of a "T" aerial.
Probably the most interesting feature of the directive action of aerials lies in the fact that a land station is able to determine the approximate bearing of a ship signaling with a horizontal aerial.
FIG. 20.—Flat top aerials of the inverted "U" and "T" types.
It is beyond the scope of the book to enter into all of the engineering details pertaining to the installation of a wireless station, but a few remarks and instructions for the benefit of those who may be interested in this phase of the subject may be appreciated.
The flat-top "T" aerial gives the best "all around" results. The vertical and umbrella forms are close seconds.
FIG. 21.—Umbrella aerial.
For the very best results, the top or horizontal portion of a "T" aerial should be slightly shorter than the vertical section.
The umbrella type of antenna is very efficient. Instead of a wooden mast, an iron pipe terminating above in a system of wires, inclining downward and serving both as part of the aerial and as guys to support the pole, is often used. The bottom of the pole is placed on an insulating base, protected from the rain by a small shelter. The wires are insulated near the lower ends by strain-insulators. The action of the wires is to serve as a capacity extension to the aerial.
FIG. 22.—An amateur aerial (flat-top).
Vertical aerials are not as efficient as either of those forms just mentioned. They require to be 50 per cent. higher than a flat-top aerial, in order to be of the same value.
The "L" and "V" types are somewhat directional. They are used where the highest point must be near the station, with a lower point some distance away. It is possible to secure excellent results with either type.
The terms straightaway and loop denote the method of connecting the aerial wires. In the first form the upper or free ends of the wires terminate at the insulators. In the loop form they are all connected together, and divided into two sections, each of which is led separately into the operating room.
FIG. 23.—Diagram showing the difference between "loop" and "straightaway" aerials.
The straightaway aerial is the most efficient in most cases, but wherever great height cannot be obtained, or the aerial is necessarily short, the loop aerial will give the best results.
Bare copper wire is the best, and is generally used for aerials. Wherever the stretch is 100 feet or over, however, so that the wires are subjected to considerable strain from their own weight, phosphor bronze is used because of its greater tensile strength. Commercial and navy stations employ stranded wire. High frequency currents have the peculiar property of traveling near the surface of wires and conductors. They do not permeate to the center of the wire, as do normal currents. The surface of a stranded wire is greater in comparison to its cross-section than a solid conductor of the same diameter, and therefore is often employed because it offers less resistance to currents of this sort.
FIG. 24.—Showing how wires are arranged and insulated.
Aluminum wire is very light, and causes very little strain on the pole or cross-arms. It offers more resistance than copper, but some of the larger sizes may be used with equally good results.
Iron wire must never be used, even if galvanized or tinned. It possesses a certain reactance tending to choke off the high frequency currents.
FIG. 25.—Aerial insulator.
The aerial is always very carefully insulated from its supports and surrounding objects by special insulators, capable of withstanding severe strains, made of a moulded material having an iron ring imbedded in each end.
FIG. 26.—Leading-in insulator.