Popular Science Monthly/Volume 56/November 1899/Wireless Telegraphy
By Prof. JOHN TROWBRIDGE,
DIRECTOR OF JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY.
I NEVER visit the historical collection of physical apparatus in the physical laboratory of Harvard University without a sense of wonderment at the marvelous use that has been made of old and antiquated pieces of apparatus which were once considered electrical toys. There can be seen the first batteries, the model of dynamo machines, and the electric motor. Such a collection is in a way a Westminster Abbey—dead mechanisms born to new uses and a great future.
There is one simple piece of apparatus in the collection, without which telephony and wireless telegraphy would be impossible. To my mind it is the most interesting skeleton there, and if physicists marked the resting places of their apparatus laid to apparent rest and desuetude, this merits the highest sounding and most suggestive inscription. It is called a transformer, and consists merely of two coils of wire placed near each other. One coil is adapted to receive an electric current; the other coil, entirely independent of the first, responds by sympathy, or what is called induction, across the space which separates the coils. Doubtless if man knew all the capabilities of this simple apparatus he might talk to China, or receive messages from the antipodes. He now, by means of it, analyzes the light of distant suns, and produces the singular X
Fig. 1.—Disposition of batteries and coils at the sending station, showing the arrangement of the vertical wire and the spark gap.
rays which enable him to see through the human body. By means of it he already communicates his thoughts between stations thousands of miles apart, and by means of its manifestations I hope to make this article on Wireless telegraphy intelligible. My essay can be considered a panegyric of this buried form—a history of its new life and of its unbounded possibilities.
For convenience, one of the coils of the transformer is placed inside the other, and the combination is called a Ruhmkorf coil. It is represented in the accompanying photograph (Fig. 1), with batteries attached to the inner coil, while the outer coil is connected to two balls, between which an electric spark jumps whenever the battery circuit is broken. In fact, any disturbance in the battery circuit—a weakening, a strengthening, or a break—provided that the changes are sudden, produces a corresponding change in the neighboring circuit. One coil thus responds to the other, in some mysterious way, across the interval of air which separates them. Usually the coils are placed very near to each other—in fact, one embraces the other, as shown in the photograph.
The coils, however, if placed several miles apart, will still respond to each other if they are made sufficiently large, if they are properly placed, and if a powerful current is used to excite one coil. Thus, by simply varying the distance between the coils of wire we can send messages through the air between stations which are not connected with a wire. This method, however, does not constitute the system of wireless telegraphy of Marconi, which it is the object of this paper to describe. Marconi has succeeded in transmitting messages over forty miles between points not connected by wires, and he has accomplished this feat by merely slightly modifying the disposition of the coils, thus revealing a new possibility of the wondrous transformer. If the reader will compare the following diagram (Fig. 2) with the photograph (Fig. 1), he will see how simple the sending apparatus of Marconi is.
S is a gap between the ends of one coil, across which an electric spark is produced whenever the current from the batteries B flowing through the coil C is broken by an arrangement at D. This break produces an electrical pulsation in the coil C’, which travels up and down the wire W, which is elevated to a considerable height above the ground. This pulsation can not be seen by the eye. The wire does not move; it appears perfectly quiescent
Fig. 2.—Diagram of the arrangement of wires and batteries at the receiving station.
and dead, and seems only a wire and nothing more. At night, under favorable circumstances, one could see a luminosity on the wire, especially at the end, when messages are being transmitted, by a powerful battery B.
It is very easy to detect the electric lines which radiate from every part of such a wire when a spark jumps between the terminals S of the coil. All that is necessary to do is to pass the wire through a sensitive film and to develop the film. The accompanying photograph (Fig. 3) was taken at the top of such a wire, by means of a very powerful apparatus at my command. When the photograph is examined with a microscope the arborescent electric lines radiating from the wire, like the rays of light Fig. 2a represents a more complete electrical arrangement of the receiver circuit. The vertical wire, W’, is connected to one wire of the coherer, L. The other wire of the coherer is led to the ground, G. The wires in the coherer, L, are separated by fine metallic particles. B represents a battery. E, an electro-magnet which attracts a piece of iron, A (armature), and closes a local battery, B, causing a click of the sounder (electro-magnet), S. The magnetic waves (Fig. 5) embracing the wire, W’, cause a pulsation in this wire which produces an electrical disturbance in the coherer analogous to that shown in Fig. 3, by means of which an electrical current is enabled to pass through the electro-magnet, E. from a star, exhibit a beautiful fernlike structure. These lines, however, are not chiefly instrumental in transmitting the electric pulse across space.
There are other lines, called magnetic lines of force, which emanate from every portion of the vertical wire W just as ripples spread out on the surface of placid water when it is disturbed by the fall of a stone. These magnetic ripples travel in the ether of space, and when they embrace a neighboring wire or coil produce similar ripples, which whirl about the distant wire and produce in some strange way an electrical current in the wire. These magnetic pulsations can travel great distances.
In the photographs of these magnetic whirls, Fig. 4 is the whirl produced in the circuit C’ by the battery B (Fig. 2), while Fig. 5 is that produced by electrical sympathy, or as it is called induction, in a neighboring wire. These photographs were obtained by passing the circuits through the sensitive films, perpendicularly to the latter, and then sprinkling very fine iron filings on these surfaces and exposing them to the light. In order to obtain these photographs a very powerful electrical current excited the coil C (Fig. 2), and the neighboring circuit W’ (Fig. 5) was placed very near the circuit W.
When the receiving wire is at the distance of several miles from the sending wire it is impossible to detect by the above method the magnetic ripples or whirls. We can, however, detect the electrical currents which these magnetic lines of force cause in the receiving wire; and this leads me to speak of the discovery of a remarkable phenomenon which has made Marconi's system of wireless telegraphy possible. In order that an electrical current may flow through a mass of particles of a metal, a mass, for instance, of iron filings, it is necessary either to compress them or to cause a minute spark or electrical discharge between the particles. Now, it is supposed that the magnetic whirls, in embracing the distant receiving circuit, cause these minute sparks, and thus enable the electric current from the battery B to work a telegraphic sounder or bell M. The metallic filings are inclosed in a glass tube
Fig. 3.—Photograph of the electric lines which emanate from the end of the wire at the sending station, and which are probably reproduced among the metallic filings of the coherer at the receiving station.
between wires which lead to the battery, and the arrangement is called a coherer. It can be made small and light. Fig. 6 is a representation in full size of one that has been found to be very sensitive. It consists of two silver wires with a few iron filings contained in a glass tube between the ends of the wires. It is necessary that this little tube should be constantly shaken up in order that after the electrical circuit is made the iron filings should return to their non-conducting condition, or should cease to cohere together, and should thus be ready to respond to the following signal. My colleague. Professor Sabine, has employed a very small electric motor to cause the glass tube to revolve, and thus to keep the filings in motion while signals are being received. Fig. 7 shows the arrangement of the receiving apparatus.
The coherer and the motor are shown between two batteries, one of which drives the motor while the other serves to work the bell or sounder when the electric wire excites the iron filings. In Fig. 2 this receiving apparatus is shown diagrammatically. B is the battery which sends a current through the sounder M and the
Fig. 4.—Magnetic whirls about sending wire.
coherer N when the magnetic whirls coming from the sending wire W embrace the receiving wire W’.
The term wireless telegraphy is a misnomer, for without wires the method would not be possible. The phenomenon is merely an enlargement of one that we are fully conscious of in the case of telegraph and telephone circuits, which is termed electro-magnetic induction. Whenever an electric current suddenly flows or suddenly ceases to flow along a wire, electrical currents are caused by induction in neighboring wires. The receiver employed by Marconi is a delicate spark caused by this induction, which forms a bridge so that an electric current from the relay battery can pass and influence magnetic instruments.
Many investigators had succeeded before Marconi in sending telegraphic messages several miles through the air or ether between two points not directly connected by wires. Marconi has extended the distance by employing a much higher electro-motive force at the sending station and using the feeble inductive effect at a distance to set in action a local battery. It is evident that wires are needed at the sending station from every point of which magnetic and electric waves are sent out, and wires at the receiving station which embrace, so to speak, these waves in the manner shown by our photographs. These waves produce minute sparks in the receiving instrument, which act like a suddenly drawn flood gate in allowing the current from a local battery to flow through the circuit in which the spark occurs, and thus produce a click on a telegraphic instrument.
We have said that messages had been sent by what is called wireless telegraphy before Marconi made his experiments. These messages had also been sent by induction, signals on one wire being received by a parallel and distant wire. To Marconi is due the credit of greatly extending the method by using a vertical wire. The method of using the coherer to detect electric pulses is not due, however, to Marconi. It is usually attributed to Branly;
Fig. 5.—Magnetic whirls about the receiving wire.
it had been employed, however, by previous observers, among whom is Hughes, the inventor of the microphone, an instrument analogous in its action to that of the coherer. In the case of the microphone, the waves from the human voice shake up the particles of carbon in the microphone transmitter, and thus cause an electrical current to flow more easily through the minute contacts of the carbon particles. The action of the telephone transmitter, which also consists of minute conducting particles in which a battery terminals are immersed, and the analogous coherer is microscopic, and there are many theories to account for their changes of resistance to electrical currents. We can not, I believe, be far wrong in thinking that the electric force breaks down the insulating effect of the infinitely thin layers of air between the particles, and thus allows an electric current to flow. This action is doubtless of the nature of an electric spark. An electric spark, in the case of wireless telegraphy, produces magnetic and electric lines of force in space, these reach out and embrace the circuit containing the coherer, and produce in turn minute sparks. Similia similibus—one action perfectly corresponds to the other.
The Marconi system, therefore, of what is called wireless telegraphy is not new in principle, but only new in practical application. It had been used to show the phenomena of electric waves in lecture rooms. Marconi extended it from distances of sixty to one hundred feet to fifty or sixty miles. lie did this by lifting the sending-wire spark on a lofty pole and improving the sensitiveness of the metallic filings in the glass tube at the receiving station.
Fig. 6.—The coherer employed to receive the electric waves. (One and a third actual size.)
He adopted a mechanical arrangement for continually tapping the coherer in order to break up the minute bridges formed by the cohering action, and thus to prepare the filings for the next magnetic pulse. The system of wireless telegraphy is emphatically a spark system strangely analogous to flash-light signaling, a system in which the human eye with its rods and cones in the retina acts as the coherer, and the nerve system, the local battery, making a signal or sensation in the brain.
Let us examine the sending spark a little further. An electric spark is perhaps the most interesting phenomenon in electricity. What causes it—how does the air behave toward it—what is it that apparently flows through the air, sending out light and heat waves as well as magnetic and electric waves? If we could answer all these questions, we should know what electricity is. A critical study of the electric spark has not only its scientific but its practical side. We see the latter side evidenced by its employment in wireless telegraphy and in the X rays; for in the latter case we have an electric discharge in a tube from which the air is removed—a special case of an electric spark. In order to understand the capabilities of wireless telegraphy we must turn to the scientific study of the electric spark; for its practical employment resides largely in its strength, in its frequency in its position, and in its power to make the air a conductor for electricity. All these points are involved in wireless telegraphy. How, then, shall we study the electric spark? The eye sees only an instantaneous flash following a devious path. It can not tell in what direction a spark flies (a flash of lightning, for instance), or indeed whether it has a direction. There is probably no commoner fallacy mankind entertains than the belief that the direction of lightning, or any electric spark, can be ascertained by the eye—that is, the direction from the sky to the earth or from the earth to the sky.
Fig. 7.—Arrangement of batteries of motor (to disturb the coherer) and the sounder by which the messages are received.
I have repeatedly tested numbers of students in regard to this question, employing sparks four to six feet in length, taking precautions in regard to the concealment of the directions in which I charged the poles of the charging batteries, and I have never found a consensus of opinion in regard to directions. The ordinary photograph, too, reveals no more than the eye can see—a brilliant, devious line or a flaming discharge.
A large storage battery forms the best means of studying electric sparks, for with it one can run the entire gamut of this phenomenon—from the flaming discharge which we see in the arc light on the street to the crackling spark we employ in wireless telegraphy, and the more powerful discharges of six or more feet in length which closely resemble lightning discharges. A critical study of this gamut throws considerable light on the problem of the possibility of secret wireless telegraphy—a problem which it is most important to solve if the system is to be made practical; for at present the message spreads out from the sending spark in great circular ripples in all directions, and may be received by any one.
Several methods enable us to transform electrical energy so as to obtain suitable quick and intense blows on the surrounding medium. Is it possible that there is some mysterious vibration in the spark which is instrumental in the effective transmission of electrical energy across space? If the spark should vibrate or oscillate to and fro faster than sixteen times a second the human eye could not detect such oscillations; for an impression remains on the eye one sixteenth of a second, and subsequent ones separated by intervals shorter than a sixteenth would mingle together and could not be separated. The only way to ascertain whether the spark is oscillatory, or whether it is not one spark, as it appears to the eye, but a number of to-and-fro impulses, is to photograph it by a rapidly revolving mirror. The principle is similar to that of the biograph or the vitoscope, in which the quick to-and-fro motions of the spark are received on a sensitive film, which is in rapid motion. One terminal of the spark gap, the positive terminal so called, is always brighter than the other. Hence, if the sensitive film is moved at right angles to the path of the discharge, we shall get a row of dots which are the images of the brighter terminal, and these dots occur alternately first on one terminal and then on the other, showing that the discharge oscillates—that is, leaps in one discharge
Fig. 8.—Photograph of electrical pulses. The interval between the pulses is one millionth of a second.
(which seems but one to the eye) many times in a hundred thousandth of a second. In practice it is found better to make an image of the spark move across the sensitive film instead of moving the film. This is accomplished by the same method that a boy uses in flashing sunlight by means of a mirror. The faster the mirror moves the faster moves the image of the light. In this way a speed of a millionth of a second can be attained. In this case the distance between the dots on the film may be one tenth of an inch, sufficient to separate them to the eye. The photograph of electric sparks (rig. 8) was taken in this manner. The distance between any two bright spots in the trail of the photographic images represents the time of the electric oscillation or the time of
Fig. 9.—Photograph of a pilot spark, which is the principal factor in the method of wireless telegraphy.
the magnetic pulse or wave which is sent out from the spark, and which will cause a distant circuit to respond by a similar oscillation.
At present the shortest time that can, so to speak, be photographed in this manner is about one two-millionth of a second. This is the time of propagation of a magnetic wave over four hundred feet long. The waves used in wireless telegraphy are not more than four feet in length—about one hundredth the length of those we can photograph. The photographic method thus reveals a mechanism of the spark which is entirely hidden from the eye and will always be concealed from human sight. It reveals, however, a greater mystery which it seems incompetent to solve—the mystery of what is called the pilot spark, the first discharge which we see on our photograph (Fig. 9) stretching intact from terminal to terminal, having the prodigious velocity of one hundred and eighty thousand miles a second. None of our experimental devices suffice to penetrate the mystery of this discharge. It is this pilot spark which is chiefly instrumental in sending out the magnetic pulses or waves which are powerful enough to reach forty or fifty miles. The preponderating influence of this pilot spark—so called since it finds a way for the subsequent surgings or oscillations—is a bar to the efforts to make wireless telegraphy secret. We can see from the photograph how much greater its strength is than that of the subsequent discharges shown by the mere brightening of the terminals. A delicate coherer will immediately respond to the influence of this pilot spark, and the subsequent oscillations of this discharge will have little effect. How, then, can we effectively time a receiving circuit so that it will respond to only one sending station? We can not depend upon the oscillatory nature of the spark, or adopt, in other words, its rate of vibration and form a coherer with the same rate.
It seems as if it would be necessary to invent some method of sending pilot sparks at a high and definite rate of vibration, and of employing coherers which will only respond to definite powerful rates of magnetic pulsation. Various attempts have been made to produce by mechanical means powerful electric surgings, but they have been unsuccessful. Both high electro-motive force and strength of current are needed. These can be obtained by the employment of a great number of storage cells. The discharge from a large number of these cells, however, is not suitable for the purpose of wireless telegraphy, although it may possess the qualifications of both high electrical pressure and strength of current.
The only apparatus we have at command to produce quick blows on the ether is the Ruhmkorf coil. This coil, I have said, has been in all our physical cabinets for fifty years. It contained within itself the germ of the telephone transmitter and the method of wire-less telegraphy, unrecognized until the present. In its elements it consists, as we have seen, of two electrical circuits, placed near each other, entirely unconnected, A battery is connected with one of these circuits, and any change in the strength of the electrical current gives a blow to the ether or medium between the two circuits. A quick stopping of the electrical current gives the strongest impulse to the ether, which is taken up by the neighboring circuit. For the past fifty years very little advance has been made in the method of giving strong electrical impulses to the medium of space. It is accomplished simply by a mechanical breaking of the connection to the battery, either by a revolving wheel with suitable projections, or by a vibrating point. All the various forms of mechanical breaks are inefficient. They do not give quick and uniform breaks. Latterly, hopes have been excited by the discovery of a chemical break, called the Weynelt interrupter, shown in Fig. 1. The electrical current in passing through a vessel of diluted sulphuric acid from a point of platinum to a disk of lead causes bubbles of gas which form a barrier to its passage which is suddenly broken down, and this action goes on at a high rate of speed, causing a torrent of sparks in the neighboring circuit. The medium between the two circuits is thereby submitted to rapid and comparatively powerful impulses. The discovery of this and similar chemical or molecular interruptions marks an era in the history of the electrical transformer, and the hopes of further progress by means of them is far greater than in the direction of mechanical interruptions.
We are still, however, unable to generate sufficiently powerful and sufficiently well-timed electrical impulses to make wireless telegraphy of great and extended use. Can we not hope to strengthen the present feeble impulses in wireless telegraphy by some method of relaying or repeating? In the analogous subject of telephony many efforts have also been made to render the service secret, and to extend it to great distances by means of relays. These efforts have not been successful up to the present. We still have our neighbors' call bells, and we could listen to their messages if we were gossips. The telephone service has been extended to great distances—for instance, from Boston to Omaha—not by relays, but by strengthening the blows upon the medium between the transmitting circuit and the receiving one, just as we desire to do in what is called wireless telegraphy, the apparatus of which is almost identical in principle to that employed in telephony. The individual call in telephony is not a success for nearly the same reasons that exist in the case of wireless telegraphy. Perfectly definite and powerful rates of vibration can not be sent from point to point over wires to which only certain definite apparatus will respond. There are so many ways in which the energy of the electric current can be dissipated in passing over wires and through calling bells that the form of the waves and their strength becomes attenuated. The form of the electrical waves is better preserved in free space, where there are no wires or where there is no magnetic matter. The difficulty in obtaining individual calls in wireless telegraphy resides in the present impossibility of obtaining sufficiently rapid and powerful electrical impulses, and a receiver which will properly respond to a definite number of such impulses.
The question of a relay seems as impossible of solution as it does in telephony. The character of speech depends upon numberless delicate inflections and harmonies. The form, for instance, of the wave transmitting the vowel a must be preserved in order that the sound may be recognized. A relay in telephony acts very much like one's neighbor in the game called gossip, in which a sentence repeated more or less indistinctly, after passing from one person to another, becomes distorted and meaningless. No telephone relay has been invented which preserves the form of the first utterance, the vowel a loses its delicate characteristics, and becomes simply a meaningless noise. It is maintained by some authorities that such a relay can not be invented, that it is impossible to preserve the delicate inflections of the human voice in passing from one circuit to another, even through an infinitesimal air gap or ether space. It is well, however, to reflect upon Hosea Bigelow's sapient advice "not to prophesy unless you know." It was maintained in the early days of the telephone that speech would lose so many characteristics in the process of transmission over wires and through magnetic apparatus that it would not be intelligible. It is certain that at present long-distance transmission of speech can only be accomplished by using more powerful transmitters, and by making the line of copper better fitted for the transmission—just as quick transportation from place to place has not been accomplished by quitting the earth and by flying through space, but by obtaining more powerful engines and by improving the roadbeds.
The hopes of obtaining a relay for wireless telegraphy seem as small as they do in telephony. The present method is practically limited to distances of fifty or sixty miles—distances not much exceeding those which can be reached by a search-light in fair weather. Indeed, there is a close parallelism between the search-light and the spark used in Marconi's experiments: both send out waves which differ only in length. The waves of the search-light are about one forty-thousandth of an inch long, while the magnetic waves of the spark, invisible to the eye, are three to four feet—more than a million times longer than the light waves. These very long waves have this advantage over the short light waves; they are able to penetrate fog, and even sand hills and masonry. One can send messages into a building from a point outside. A prisoner could communicate with the outer world, a beleaguered garrison could send for help, a disabled light-ship could summon assistance, and possibly one steamer could inform another in a fog of its course.
Wireless telegraphy is the nearest approach to telepathy that has been vouchsafed to our intelligence, and it serves to stimulate our imagination and to make us think that things greatly hoped for can be always reached, although not exactly in the way expected. The nerves of the whole world are, so to speak, being bound together, so that a touch in one country is transmitted instantly to a far-distant one. Why should we not in time speak through the earth to the antipodes? If the magnetic waves can pass through brick and stone walls and sand hills, why should we not direct, so to speak, our trumpet to the earth, instead of letting its utterances skim over the horizon? In regard to this suggestion, we know certainly one fact from our laboratory experiences: that these magnetic waves, meeting layers of electrically conducting matter, like layers of iron ore, would be reflected back, and would not penetrate. Thus a means may be discovered through the instrumentality of such waves of exploring the mysteries of the earth before success is attained in completely penetrating its mass.