Fig. 4.Fig. 4.In this diagram, 1 and 2 are tuned reeds;1A 2Aare receivers tuned to the reeds 1 and 2 respectively; 1 and1Aare in unison, also 2 and2A, but the two groups (the 1s and the 2s) differ from each other in pitch.
In this diagram, 1 and 2 are tuned reeds;1A 2Aare receivers tuned to the reeds 1 and 2 respectively; 1 and1Aare in unison, also 2 and2A, but the two groups (the 1s and the 2s) differ from each other in pitch.
There were two ways of reading by the harmonic method. One was by the long and short tone-sounds and the other by the ordinary sounder.
The vibration of the receiving-reed was made to open and close a local circuit like a common Morse relay and thus operate the sounder. It is useless to try to send a message if the sender and receiver are out of tune with each other in this system.
What is true in science is true in life. If we are out of tune with our surroundings we only beat the air, and our efforts are in vain. We get no sympathetic response.
A novel form of double transmission was invented by the writer soon after the completion of the harmonic system, and was an outgrowth of it. It is still in use on some of the railroad-lines. An ordinary railroad telegraph-line has an instrument in circuit in every office along the road, chiefly for purposes of train-dispatching. As we have heretofore explained, whenever any one office is sending, the dispatch is heard in all of the offices. The "Way duplex" system permits of the use of the line for through business simultaneously with the operation of the local offices. That is to say, any station along the line may be telegraphing with any other station by the ordinary Morse method, and at the same time messages may be passing back and forth between the two end offices.
This is accomplished by the following method: At each end of the line there is a tuned reed, such as we have described in our last chapter, that is kept constantly in vibration by a local battery during working hours.This vibrator is so arranged in relation to the battery that whenever the key belonging to it is depressed the current all through the line is rendered vibratory. There is also in circuit at each end of the line a harmonic relay, that is tuned in accord with the vibrating reed of the sender. If either key belonging to this part of the system is opened, as in the act of sending a message, these harmonic relays, being tuned in sympathy with the sending-vibrator, will respond, thus sending Morse characters made up of a tone broken into dots and dashes. This tone can be read directly from the relay, or, as is usually the case, it causes the sounder to operate in the common way.
You will at once inquire why the ordinary Morse instruments in the local offices are not affected by these vibratory signals, and also why the harmonic instruments at the end office are not affected by the working of the local offices. The local office does not open the circuit entirely, but simply cuts out a resistance by the operation of the special harmonic key. When a resistance is thrown into an electric circuit it weakens the current in proportion to the amount of resistance interposed. You will see that there is some current still left in the line when the key is open, but the spring of the relay at the local office is so adjusted as to pull the armatures away from the magnets whenever the current is weakened bythrowing in the resistance, so that by this means an ordinary Morse telegraphic relay may be worked without ever entirely opening the circuit. In the Way duplex system there is a resistance at each station that is cut in and out by the operation of its key, which causes all the instruments in the line to work simultaneously except the two harmonic relays located one at each end of the line. These will not respond to anything but the vibratory signal.
In order to prevent the Morse relays at the local offices from responding to the vibratory current a condenser is connected around them. This condenser serves two purposes: It enables the short impulses of the vibrating current to pass around the relays without having to be resisted by the coils of the magnets, and between the pulsations each condenser will discharge through the relay at the local offices, and thus fill in the gap between the pulsations, producing the effect on the relay of a steady current. When a line is thus equipped it may be treated in every respect as two separate wires, one of them doing way business and the other through business. It is a curious blending of science and mechanism.
Another interesting application was made of the system of transmission by musical tones—by Edison, some years ago. We refer to the transmission of messages to and from a moving railroad-train with the head office at the end of the line. In this case the message was transmitted a part of the distance through the air;—another instance of wireless telegraphy. The operation was as follows: One of the wires strung on the poles nearest to the track was fitted up with a vibrator and key at the end of the line similar to that of the Way duplex just described. In one of the cars was another battery, key and vibrator, and as only one tone was used, no tone-selecting device or harmonic relay was needed, but instead an ordinary receiving-telephone was used to read the long and short sounds sent over the lines. One end of the battery in the car was connected through the wheels to the earth, while the other end was connected to the metal roof of the car. Being thus equipped, we will suppose our train to be out on the road forty or fifty miles from either end of the line, moving at the rate of forty miles an hour. The operator at Chicago, say, wishes to send a message to the moving train; he operates his key in the ordinary manner, which makes the current on the line vibratory during the time the key is depressed. These electrical vibrations cause magnetic vibrations, or ether-waves, to radiate in every direction from the wire, at right angles to the direction of the current, like rays of light. When they strike the roof of the car they create electrical impulses in the metalby induction (described in Chap. VI). These impulses pass through a telephone located in the car to the ground. A Morse operator listening, with the telephone to his ear, will hear the message through the medium of a musical tone chopped up into the Morse code. In like manner the operator in the car may transmit a message to the roof of the car and thence through the air to the wire, which will be heard, by any one listening, in a telephone which is connected in that circuit,—and, as a matter of fact, it will be heard from any wire that may be strung on any of the poles on either side of the road.
Some years ago an experiment of this kind was made on one of the roads between Milwaukee and Chicago.
What wonderful things can be done with electricity! As a servant of man it is reliable and accurate—seeming almost to have the qualities of docility—when under intelligent direction, that is in accord with the laws of nature; but under other conditions it changes from the willing servant to a hard master, hesitating not to destroy life or property without regard to persons or things.
In the foregoing chapters I have described the method of transmitting musical tones telegraphically and its applications to multiple telegraphy, as well as to a mode of communicating with a moving railroad-train. As I stated in a former chapter, after discovering a method of transmitting harmony as well as melody, I had in mind two lines of development, one in the direction of multiple telegraphy, and the other that of the transmission of articulate speech. I will not attempt to give the names of all the people who have contributed to the development of the telephone (as this alone would fill a volume) but only describe my own share in the work—leaving history to give each one due credit for his part. While I do not intend, here, to enter into any controversy regarding the priority of the invention of the telephone, I wish to say that from the time I began my researches, in the winter of 1873-4, until some time after I had filed my specification for a speaking or articulating telephone, in the winter of 1875-76, I had no idea that any one else had done or was doing anything in this direction. I wish to say further that if I had filed my description of a telephone as an application for a patent instead of as a caveat, and had prosecuted it to a patent, without changing a word in the specification as it stands to-day, I should have been awarded the priority of invention by the courts. I am borne out in this assertion by the highest legal authority. In law, acaveat(Latin word, meaning "Let him beware") is a warning to other inventors, to protect an incomplete invention; whereas in fact the invention to be protected may be complete. Anapplicationfor a patent is presumed by the law to be for a completed invention; but it may be, and very often is, incomplete. It would often make a very great difference if decisions were rendered according to the facts in the case rather than according to rules of law and practice, that sometimes work great injustice to individuals.
As has been said in another chapter, in the summer of 1874 I went to Europe in the interest of the telephone, taking my apparatus, as then developed, with me. I came home early in the fall and resumed my experimental work. Many interesting as well as amusing things occurred during these experiments.
I remember that in the fall or early winter of 1874 I was in Milwaukee with my apparatuscarrying on some experiments on a wire between Milwaukee and Chicago. I had my musical transmitter along, and one evening, for the entertainment of some friends at the Newhall House, a wire was stretched across the street from the telegraph office into one of the rooms of the hotel. A great number of tunes were played at the telegraph-office by Mr. Goodridge, who was my assistant at that time, which were transmitted across the street, as before stated. In those days it was a common practice in telegraphy to use one battery for a great number of lines. For instance, starting with one ground-wire which connected with, say, the negative pole of the battery, from the positive pole two, three or a half-dozen lines might be connected, running in various directions, connecting with the ground at the further end, thus completing their circuits. For use in transmitting tones across the street that evening we connected our line-wire on to the telegraph company's battery, which consisted of 100 or more cells, and which had four or five more lines radiating from the end of the battery to different parts of Wisconsin. Our line was tapped on to the battery (without changing any of its connections) twenty cells from the ground-wire. In transmitting, each vibration would momentarily shut off these twenty cells from the lines that were connected with the wholebattery. The effect of this (an effect that we did not anticipate at the time) was to send a vibratory current out on all the lines that were connected with that single battery as well as across the street. A great many familiar tunes were played during the course of an hour or two which, unconsciously for us, were creating great consternation throughout the State of Wisconsin, in many of the offices through which these various lines passed.
Next morning reports and inquiries began to come in from various towns and cities west, northwest and north, giving details of the phenomena that were noticed on the instruments located in the various offices along the lines. They reported their relays as singing tunes; one party said he thought the instruments were holding a prayer-meeting from the fact that they seemed to be singing hymn-tunes for quite a while, but this notion was finally dissipated, because they grew hilarious and sang "Yankee Doodle."
One operator, up in the pine woods of northern Wisconsin, did not seem to take the cheerful view of it that some of the others did. He was sitting alone in the telegraph-office that evening when he thought he heard the notes of a bugle in the distance; he got up and went to the door to listen, but could hear nothing; but on coming back into the room he heard the same bugle notes veryfaintly. He was inclined to be somewhat superstitious and grew very nervous; finally, on looking around, he located the sound in his relay, but this did not help matters with him. With superstitious awe he listened to the instrument for a few moments, while it gave out the solemn tones of "Old Hundred," then it suddenly jumped into a hilarious rendering of "Yankee Doodle." This was too much for our nervous friend, and hastily putting on his overcoat, he left the office for the night.
On another occasion, when I was giving a lecture in one of the cities outside of Chicago, where exhibitions of music transmitted from Chicago were given, one of the operators along the line was very much astonished by his switchboard suddenly becoming musical. Orders had been given for the instruments in all the local offices to be cut out of the particular line that I was using. Hence the instrument in this particular office was not in the circuit through which the tunes were being transmitted. The wire, however, ran through his switchboard, and owing probably to a loose connection, or an induced effect, there was a spark that leaped across a short space at each electrical pulsation that passed through the line, thus reproducing the notes of the various tunes played.
You will remember in one of the chapters on sound (Volume II.), it is stated that amusical tone is made up of a succession of sounds repeated at equal intervals, and that the pitch of the tone is determined by the number of sound-impulses per second. Applying this law to the sparks, you will be able to see how the switchboard played tunes for the operator.
In the foregoing experiments in transmitting musical tones telegraphically, I used a great many different varieties of receivers. Some of them were designed with metal diaphragms mounted over single electromagnets, not unlike the receiver of an ordinary telephone. These instruments would both transmit and receive articulate speech when placed in circuit with the right amount of battery to furnish the necessary magnetism. However, they were not used in that way at the time they were first made—in 1874. These I called common receivers, as they were designed to reproduce all tones equally well. I designed and constructed another form of receiver, based somewhat upon the theory of the harmonic telegraph.
This consisted of an electromagnet of considerable size, mounted upon a wooden rod about ten feet long. Mounted upon this rod were also resonating boxes or tubes made of wood of the right size to have their air-cavities correspond with the various pitches of the transmitting-reeds, so that each tone would bere-enforced by some one of these air-cavities, thus giving a louder and more resonant effect to the musical notes.
Here were two types of receiver, one that would receive one sound as well as another, but none of them so loud, while the other was constructed on the principle of selection and re-enforcement, so that a particular note would be sounded by the box having a cavity corresponding to the pitch of the tone, and was much louder and of much better quality than I could get from the diaphragm receiver. One of these receivers pointed to the harmonic telegraph and the other to the speaking telephone. I knew that I had a receiver that would reproduce articulate speech or anything else that could be transmitted.
My first conceptions of an articulate speech-transmitter were somewhat complicated. I conceived of a funnel made of thin metal having a great number of little riders, insulated from the funnel at one end and resting lightly in contact with the funnel at the other end. These riders were to be made of all sizes and weights so as to be responsive to all rates of vibration. In the light of the present day we know that such an arrangement would have transmitted articulate speech, but perhaps not so well as a single point would do when properly adjusted. My mind clung to this idea till in the fall of 1875, when an observation Imade upon the street changed the whole course of my thinking and solved the problem. The incident I refer to took place in Milwaukee, where I was then experimenting. One day while out on an errand I noticed two boys with fruit-cans in their hands having a thread attached to the center of the bottom of each can and stretched across the street, perhaps 100 feet apart. They were talking to each other, the one holding his mouth to his can and the other his ear. At that time I had not heard of this "lovers' telegraph," although it was old. It is said to have been used in China 2000 years ago.
The two boys seemed to be conversing in a low tone with each other and my interest was immediately aroused. I took the can out of one of the boy's hands (rather rudely as I remember it now), and putting my ear to the mouth of it I could hear the voice of the boy across the street. I conversed with him a moment, then noticed how the cord was connected at the bottom of the two cans, when, suddenly, the problem of electrical speech-transmission was solved in my mind. I did not have an opportunity immediately to construct an instrument, as I had a partner who was furnishing money for the development of the harmonic telegraph and would not listen to any collateral experiments. I remember sitting down by this partner one day and telling him what Icould do in the way of transmitting speech through a wire. I told him I thought it would be very valuable if worked out. He gave me a look that I shall never forget, but he did not say a word. The look conveyed more meaning than all the words he could have said, and I did not dare broach the subject again.
However, as soon as I found opportunity, without saying a word to anybody except my patent lawyer, I filed a description, accompanied by drawings, of a speaking telephone which stands in history to-day as the first complete description on record of the operation of the speaking telephone. It described an apparatus which, when constructed, worked as described, and it is a matter of history that the first articulate speech electrically transmitted in this country was by a transmitter constructed on the principle described, and almost identically after the drawings in my caveat. While the transmitter described in this caveat was not the best form, it would transmit speech, and it contained the foundation principle of all the telephone transmitters in use to-day.
There are two methods of transmitting speech. One is known as the magneto method and the other that of varying the resistance of the circuit. My first transmitter was devised on the latter principle.
I append to this extracts from my specification filed Feb. 14, 1876:
To All Whom It May Concern:—Be it known that I, Elisha Gray of Chicago, in the County of Cook and State of Illinois, have invented a new art of transmitting vocal sounds telegraphically, of which the following is a specification: It is the object of my invention to transmit the tones of the human voice through a telegraphic circuit, and reproduce them at the receiving-end of the line, so that actual conversations can be carried on by persons at long distances apart. I have invented and patented methods of transmitting musical impressions or sounds telegraphically, and my present invention is based upon a modification of the principle of said invention, which is set forth and described in letters patent of the United States, granted to me July 27, 1875, respectively numbered 166,095 and 166,096, and also in an application for letters patent of the United States, filed by me, Feb. 23, 1875. * * * My present belief is that the most effective method of providing an apparatus capable of responding to the various tones of the human voice is a tympanum, drum, or diaphragm, stretched across one end of the chamber, carrying an apparatus for producing fluctuations in the potential of the electric circuit and consequently varying in its power. * * * The vibrations thus imparted are transmitted through an electric circuit to the receiving-station, in which circuit is included an electromagnet of ordinary construction, acting upon a diaphragm to which is attached a piece of soft iron, and which diaphragm is stretched across a receiving vocalizing chamberC, somewhat similar to the corresponding vocalizing chamberA.The diaphragm at the receiving-end of the line is thus thrown into vibrations corresponding with those at the transmitting-end, and audible sounds or words are produced.The obvious practical application of my improvement will be to enable persons at a distance to converse with each other through a telegraphic circuit, just as they now do in each other's presence, or through a speaking-tube.I claim as my invention the art of transmitting vocal sounds or conversations telegraphically through an electric circuit.
To All Whom It May Concern:—Be it known that I, Elisha Gray of Chicago, in the County of Cook and State of Illinois, have invented a new art of transmitting vocal sounds telegraphically, of which the following is a specification: It is the object of my invention to transmit the tones of the human voice through a telegraphic circuit, and reproduce them at the receiving-end of the line, so that actual conversations can be carried on by persons at long distances apart. I have invented and patented methods of transmitting musical impressions or sounds telegraphically, and my present invention is based upon a modification of the principle of said invention, which is set forth and described in letters patent of the United States, granted to me July 27, 1875, respectively numbered 166,095 and 166,096, and also in an application for letters patent of the United States, filed by me, Feb. 23, 1875. * * * My present belief is that the most effective method of providing an apparatus capable of responding to the various tones of the human voice is a tympanum, drum, or diaphragm, stretched across one end of the chamber, carrying an apparatus for producing fluctuations in the potential of the electric circuit and consequently varying in its power. * * * The vibrations thus imparted are transmitted through an electric circuit to the receiving-station, in which circuit is included an electromagnet of ordinary construction, acting upon a diaphragm to which is attached a piece of soft iron, and which diaphragm is stretched across a receiving vocalizing chamberC, somewhat similar to the corresponding vocalizing chamberA.
The diaphragm at the receiving-end of the line is thus thrown into vibrations corresponding with those at the transmitting-end, and audible sounds or words are produced.
The obvious practical application of my improvement will be to enable persons at a distance to converse with each other through a telegraphic circuit, just as they now do in each other's presence, or through a speaking-tube.
I claim as my invention the art of transmitting vocal sounds or conversations telegraphically through an electric circuit.
This specification was accompanied by cuts of the transmitter and receiver connected by a line-wire and showing one person talking to the transmitter and another listening at the receiver. These cuts may be seen in various books on the subject of telephony.
Everybody knows what the telephone is because it is in almost every man's house. But while everybody knows what it is, there are very few (comparatively speaking) that know how it works. If you remember what has been said about sound and electromagnetism it will not be hard to understand.
When any one utters a spoken word the air is thrown into shivers or vibrations of a peculiar form, and every different sound has a different form. Therefore, every articulate word differs from every other word, not only as a shape in the air, but as a sensation in the brain, where the air-vibrations have been conducted through the organ of hearing; otherwise we could not distinguish between one word and another. Every different word produces a different sensation because there is a physical difference, as a shape or motion, in the air where it is uttered. If one word contains 1000 simultaneous air-motions and another 1500 you can see that there is a physical or mechanical difference in the air.
The construction of the simplest form of telephone is as follows: Take a piece of iron rod one-half or three-quarters of an inch long and one-quarter inch thick, and after putting a spool-head on each end to hold the wire in place wind it full of fine insulated copper wire; fasten the end of this spool to the end of a straight-bar permanent magnet. Then put the whole into a suitable frame, and mount a thin circular diaphragm (membrane or plate) of iron or steel, held by its edges, so that the free end of the spool will come near to but not touch the center of the diaphragm. This diaphragm must be held rigidly at the edges.
Now if the two ends of the insulated copper wires are brought out to suitable binding-screws the instrument is done.
The permanent steel magnet serves a double purpose. When the telephone was first used commercially, the instrument now used as a receiver was also used as a transmitter. As a transmitter it is a dynamo-electric machine. Every time the iron diaphragm is moved in the magnetic field of the pole of the permanent magnet, which in this case is the free end of the spool (the iron of the spool being magnetic by contact with the permanent magnet), there is a current set up in the wire wound on the spool; a short impulse, lasting only as long as the movement lasts. The intensity ofthe impulse will depend upon the amplitude and quickness of the movement of the diaphragm. If there is a long movement there will be a strong current and vice versa. If a sound is uttered, and even if the multitude of sounds that are required to form a word, be spoken to the diaphragm, the latter partakes in kind of the air-motions that strike it. It swings or vibrates in the air, and if it is a perfect diaphragm it moves exactly as the air does, both as to amplitude and complexity of movement. You will remember that in the chapter on sound-quality (Vol. II) it was said that there were hundreds and sometimes thousands of superposed motions in the tones of some voices that gave them the element we call quality.
All these complex motions are communicated by the air to the diaphragm, and the diaphragm sets up electric currents in the wire wound on the spool, corresponding exactly in number and form, so that the current is molded exactly as the air-waves are. Now, if we connect another telephone in the circuit, and talk to one of them, the diaphragm of the other will be vibrated by the electric current sent, and caused to move in sympathy with it and make exactly the same motions relatively, both as to number and amplitude.
It will be plain that if the receiving diaphragm is making the same motions as thetransmitting diaphragm, it will put the air in the same kind of motion that the air is in at the transmitting end, and will produce the same sensation when sensed by the brain through the ear. If the air-motion is that of any spoken word it will be the same at both ends of the line, except that it will not be so intense at the receiving-end; it is the same relatively. And this is how the telephone talks.
I have said that the permanent magnet had two functions. In the case of the transmitter it is the medium through which mechanical is converted into electrical energy. It corresponds to the field-magnet of the dynamo, while the diaphragm corresponds to the revolving armature, and the voice is the steam-engine that drives it. In the second place, it puts a tension on the diaphragm and also puts the molecules of the iron core of the magnet in a state of tension or magnetic strain, and in that condition both the molecules and the diaphragm are much more sensitive to the electric impulses sent over the wire from the transmitter. This fact was experimented upon by the writer as far back as 1879 and published in the Journal of the American Electrical Society. At the present day this form of telephone is used only as a receiver.
Transmitters have been made in a variety of forms, but there are only two genericmethods of transmission. One is the magneto method—the one we have described—and the other is effected by varying the resistance of a battery current. The former will work without a battery, as the voice acting on the wire around the magnet through the diaphragm creates the current; in the latter the current is created by the battery but molded by the voice. In the latter method the current passes through carbon contacts that are moved by the diaphragm. Carbon is the best substance, because it will bear a wider separation of contact without actually breaking the current. When carbon points are separated that have an electric current passing through them, there is an arc formed on the same principle as the electric arc-light.
Great improvements in details have been made in the telephone since its first use, but no new principles have been discovered as applied to transmission.
We have spoken in another place regarding the various claimants to the invention of the telephone, but here is one that has been overlooked. A young man from the country was in a telegraph-office at one time and was left alone while the operator went to dinner. Suddenly the sounder started up and rattled away at such a rate that the countryman thought something should be done. He leaned down close to the instrument and shouted as loudlyas possible these words: "The operator has gone to dinner." From what we know now of the operation of the telephone I have no doubt but that he transmitted his voice to some extent over the wire. This young man's claims have never been put forward before, and we are doing him tardy justice. But his claim is quite as good as many others set forth by people who think they invent, whenever it occurs to them that something new might possibly be done, if only somebody would do it. And when that somebody does do it they lay claim to it.
In the early days of the telephone it was not supposed that a vocal message could be transmitted to a very great distance. However, as time went on and experiments were multiplied the distance to which one could converse with another through a wire kept on increasing.
In these days, as every one knows, it is a daily occurrence that business men converse with each other, telephonically, for a distance of 1000 miles or more; in fact, it is possible to transmit the voice through a single circuit about as great a distance as it is possible to practically telegraph. This leads us to speak of another telegraphic apparatus which we have not heretofore mentioned, and that is the telegraphic repeater. It is a common notion that messages are sent through a single circuit across the continent, but this is not thecase, although the circuits are very much longer than they were some years ago. The repeater is an instrument that repeats a message automatically from one circuit to another. For instance, if Chicago is sending a message to New York through two circuits, the division being in Buffalo, the repeater will be located at Buffalo and under the control of both the operator at Chicago and the operator in New York. When Chicago is sending, one part of the repeater works in unison with the Chicago key and is the key to the New York circuit, which begins at Buffalo. When New York is sending the other part of the repeater operates, which becomes a key which repeats the message to the Chicago line. In this way the practical result is the same as though the circuit were complete from New York to Chicago. At the present day some of the copper wires and perhaps some of the larger iron wires are used direct from Chicago to New York without repetition, but all messages between New York and San Francisco are automatically repeated at least twice and under certain conditions of weather oftener. I can remember that in wet weather in the old days, with such wires as they had then (being No. 9 iron with bad joints, which gave the circuit a high resistance) that these repeaters would be inserted at Toledo, Cleveland, Buffalo and Albany in order to work from Chicago to NewYork. Under such conditions the transmission would necessarily be slow, because an armature time will be lost at each repeater. Regarding each repeater as a key, when Chicago depresses his key the armature of the next repeater must act, and then the next successively, and all of this takes time, although only a small fraction of a second.
The repeater was a very delicate instrument and had to be handled by a skilled operator. Every wire must be in its place or the instrument would fail to operate. I remember on one occasion in Cleveland that along in the middle of the night the repeater failed to work. The operator knew nothing of the principle of its operation, so that when it failed he had to appeal to some of his superiors.
At this time there was no one in the office who knew how to adjust it, so they had to send up to the house of the superintendent and arouse him from his sleep and bring him down to the office. He looked under the table and found that one of the wires had loosened from its binding-post and was hanging down. He said immediately, "Here's the trouble; I should think you could have seen it yourself." The operator replied, "I did see that, but I didn't think one wire would make any difference." He learned the lesson that all electricians have had to learn—that even one wire makes all the difference in the world. Butthis operator was no worse in that respect than some of his superiors. One of the heads of the Cleveland office at one time in the early days wanted to give some directions to the office at Buffalo. He told the operator at the key to tell Buffalo so and so, when the operator replied: "I can't do it; Buffalo has his key open." The official immediately said with severity: "Tell him to close it." He forgot that it would be as difficult for him to tell him to close it, as it would have been to have sent the original message.
But let us go back to the telephone. While it is possible to send a message from New York to San Francisco by telegraph, it is not possible to telephone that distance, because as yet no one has been able to devise a repeater that will transfer spoken words from one line to another satisfactorily. But unless the printer and publisher bestir themselves some one may accomplish the feat before this little book reaches the reader. If this proves to be true, let the writer be the first to congratulate the successful inventor.
The first attempts at transmitting messages through wires laid in water were made about 1839. These early experiments were not very successful, because the art of wire-insulation had not attained any degree of perfection at that time. It was not until gutta-percha began to be used as an insulator for submarine lines that any substantial progress was made.
The first line, so history states, that was successfully laid and operated was across the Hudson River in 1848. This line was constructed for the use of the Magnetic Telegraph Company.
In the following year experiments with gutta-percha insulation were successfully made, and about 1850 a cable was laid across the English Channel between Dover and Calais (twenty-seven miles), consisting of a single strand of wire having a covering of gutta-percha. The insulation was destroyed in a day or two, which demonstrated the fact that all submarine cables must be protected bysome kind of armor. In 1851 another cable was laid between these two points, containing four conductors insulated with gutta-percha, and over all was an armor of iron wire. Twenty-one years later this cable was still working, and for all we know is working now. After this successful demonstration other cables were laid for longer distances.
These short-line cables served to demonstrate the relative value of different material for insulating purposes under water, and it has been found that gutta-percha possesses qualities superior to almost every other material as an insulator for submarine cables, although there are many better materials for air-line insulation. Gutta-percha when exposed to air becomes hardened and will crack, but under water it seems to be practically indestructible.
Ocean telegraphy really dates from the laying of the first successful Atlantic cable. There were many problems that had to be solved, which could be done only by the very expensive experiment of laying a cable across the Atlantic Ocean. In the first place a survey had to be made of the bottom of the ocean between the shores of America and Great Britain. The most available route was discovered by Lieutenant Maury of the United States Navy, who made a series of deep-sea soundings, and discovered that, from Newfoundland to the west coast of Ireland the bottom of the ocean was comparatively even, but gradually deepening toward the coast of Ireland until it reached a depth of 2000 fathoms. It was not so deep but that the cable could be laid on the bottom, nor so shallow as to be in danger of the waves, icebergs or large sea-animals.
The water below a certain depth is always still and not affected by winds or ocean currents. At many other points in crossing the ocean, high mountains and deep valleys are encountered, possessing all the topographical features of dry land—as the ocean bed is only a great submerged continent.
The beginning of the laying of the first Atlantic cable was on Aug. 7, 1857. On the morning of Aug. 7, 1858, a year later, after a series of mishaps and adverse circumstances that would have discouraged most men, the country was electrified by a dispatch from Cyrus W. Field of New York (to whom the final success of the Atlantic cable is mainly due), that the cable had been successfully laid and worked. But this cable worked only from the 10th of August to the 1st of September, having sent in that time 271 messages. The insulation became impaired at some point, when an attempt to force the current through by means of a large battery only increased the difficulty.
The failure of this first cable served to teach manufacturers and engineers how to construct cables with reference to the conditions under which they are to be used. It was found that in the deep sea a much smaller and less expensive cable could be used than would answer at the shore ends, where the water is shallow. The shore ends of an ocean cable are made very large, as compared to the deep-sea portions, so as to resist the effect of the waves and other interfering obstacles. It was further learned that the most successful mode of transmitting signals through the cable was with a small battery of low voltage, and by the use of very delicate instruments for receiving the messages. It is not possible to employ such instruments on cables as are used on land-lines, while it would not be a difficult feat to transmit even twice the distance over land-lines strung on poles, using the ordinary Morse telegraph.
The water of the ocean is a conductor, as well as the heavy armor that surrounds the insulation of the cable. When a current is transmitted through the conducting wires, in the center of the cable, they set up a countercharge in the armor and the water above it, somewhat as an electrified cloud will induce a charge in the earth under it, of an opposite nature. This countercharge, being so close to the conducting wire, has a retarding effectupon the current transmitted through it. An ordinary land-line that is strung on poles that are high up from the ground has this effect reduced to a minimum, but the greater the number of wires clustered together on the same poles the more difficult it becomes to send rapid signals through any one of them.
The instrument used for receiving cable messages was devised by Sir William Thompson, now Lord Kelvin. One form consists of a very short and delicate galvanometer-needle carrying a tiny mirror. This mirror is so related to a beam of light thrown upon it that it reflects it upon a graduated screen at some distance away, so that its motions are magnified many hundred times as it appears upon the screen. An operator sits in a dark room with his eye on the screen and his hand upon the key of an ordinary Morse instrument. He reads the signal at sight, and with his key transmits it to a sounder, which may be in another room, where it is read and copied by another operator. Another form of receiving-instrument carries, instead of the mirror, a delicate capillary glass tube that feeds ink from a reservoir, and by this means the movements of the needle are recorded on a moving strip of paper. The symbols (representing letters) are formed by combinations of zigzag lines. This instrument is the syphon-recorder.
Early in the history of the telegraph short lines began to be used for private purposes, and as the Morse code was familiar only to those who had studied it and were expert operators on commercial lines, some system had to be devised that any one with an ordinary English education could use; as the expense of employing two Morse operators would be too great for all ordinary business enterprises. These short lines are called private lines, and the instruments used upon them were called private-line telegraph-instruments. Of course they are now nearly all superseded by the telephone, but they are a part of history.
One of the earliest forms of short-line instruments was called the dial-telegraph. One of the first inventors, if not the first, of this form of instrument was Professor Wheatstone of England, who perfected a dial-telegraph-instrument about the year 1839. The receiving-end of this instrument consisted of a lettered dial-face, under which was clockwork mechanism and an escape-wheel controlled by an electromagnet. Each time the circuit was opened or closed the wheel would move forward one step, and each step represented one of the letters of the alphabet, so that the wheel, like the type-wheel of a printing telegraph, had fourteen teeth, each tooth representing two steps. As the reciprocating movement of the escapement had a pallet or check-piece on each side of the wheel, its movement was arrested twenty-eight times in each revolution. These twenty-eight steps correspond to the twenty-six letters of the alphabet, a dot and a space. On the shaft of the escape-wheel is fastened a hand or pointer, which revolves over a dial-face having the twenty-six letters of the alphabet, also a dot and space. The pointer was so adjusted that when the escape-wheel was arrested by one of the pallets it would stop over a letter, showing thus, letter by letter, the message which the sender was spelling out.
The transmitter consisted of a crank with a knob and a pointer on it, which was mounted over a dial that was lettered in the same way as the face of the receiving-instrument. A revolution of this crank would break and close the circuit twenty-eight times; that is to say, there were fourteen breaks and fourteen closes of the circuit. If now the transmitting-pointer and the receiving-pointer are unified so that they both start from the same point onthe dial, and the transmitting-crank is rotated from left to right, the receiving-pointer will follow it up to the limit of its speed. In transmitting a message the sender would turn his crank, or pointer, to the first letter of the word he wished to transmit, making a short pause, and then move on to the next letter, and so on to the end of the message, making a short pause on each letter. The end of a word was indicated by turning the pointer to the space-mark on the dial. The receiving-operator would read by the pauses of the needle on the various letters. This was a system of reading by sight.
There have been many forms of this dial-telegraph worked out by different inventors at different times, and quite a number of them were used in the old days. It was a slow process of telegraphing, but it was suited to the age in which it flourished. One of the difficulties of a dial-telegraph consisted in the readiness with which the transmitter and receiver would get out of unison with each other; and when this happened of course a message is unintelligible, and you have to stop and unify again.
About 1869 the writer invented a dial-telegraph to obviate this difficulty. In this system a transmitter and receiver were combined in one instrument, and instead of a crank there were buttons arranged around the dial in acircle, one opposite each letter. When not in operation the pointers of both instruments at both stations stood at zero. In the act of transmitting the operator would depress the button opposite the letter he wished to indicate, when immediately the pointers of both instruments would start up and move automatically, step by step, until the pointer came in contact with the stem of the depressed button, when it would be arrested, and at the same time cut out the automatic transmitting-mechanism and cause both needles to remain stationary during the time the button was depressed. Upon releasing the button the pointers both fall back to zero at one leap.
The first private line equipped by this instrument was for Rockefeller, Andrews & Flagler, which was the firm name of the parties who afterward organized the Standard Oil Company. This line was built between their office on the public square in Cleveland and their works over on the Cuyahoga flats.
It seemed, however, to be the fate of the writer to make new inventions that would supersede the old ones before they were fairly brought into use. Very soon after the dial-telegraph began to be used, printing telegraph instruments for private-line purposes superseded them. About 1867 a printing instrument was devised for stock reporting, which in one of its forms is still in use. Soon afterthe invention of this form of printer a company was organized to operate not only these stock-reporting lines, but short lines for all sorts of private purposes. Following the invention of the stock-reporting instrument there were several adaptations made of the printing telegraph for private-line purposes. Among others the writer invented one known as "Gray's automatic printer," a cut and a description of which may be found on page 684 in "Electricity and Electric Telegraph," by George B. Prescott, published in 1877. This instrument was adopted by the Gold and Stock Telegraph Company as their standard private-line printer. It was first introduced in the year 1871, and at the time the telephone began to be used there were large numbers of these printers in operation in all of the leading cities and towns in the United States. While this has been superseded to a large extent by the telephone, there are still a few isolated cases where it is used.
Short lines have multiplied for all sorts of purposes, until to-day the money invested in them largely exceeds the amount invested in the regular commercial telegraphic enterprises.
The invention of the telephone created such a demand for short-line service that some scheme had to be devised not only to make room for the necessary wires, but to so cheapen the instruments as to bring them within reachof the ability of the ordinary man of business.
This problem has been solved (but not without many difficulties) by the inauguration of what is known as the "central station." By this system one party simply controls a single wire from his office or residence to the central station; here he can have his line connected with any other wire running into this same station, by calling the central operator and asking for the required number. It is useless to tell the public that very often this number is "busy," and here is the great drawback to the central-station system. This is especially true in large cities, where there are a great number of lines. The switchboards in large cities are necessarily very complicated affairs, and it requires a number of operators to answer the many calls that are constantly coming in. Each central-station operator presides over a certain section of the board, and as this section has to be related in a certain way to every other section, it is easy to see wherein arises the complication.
In large cities the central stations themselves have to be divided and located in different districts, being connected by a system of trunk lines.
So far we have described several methods of electrical communication at a distance, including the reading of letters and symbols at sight (as by the dial-telegraph and the Morse code embossed on a strip of paper); printed messages and messages received by means of arbitrary sounds, and culminating in the most wonderful of all, the electrical transmission of articulate speech.
None of these systems, however, are able to transmit a message that completely identifies the sender without confirmation in the form of an autograph letter by mail.
In 1893 there was exhibited in the electrical building at the World's Fair an instrument invented by the writer called the Telautograph. As the word implies, it is a system by which a man's own handwriting may be transmitted to a distance through a wire and reproduced in facsimile at the receiving-end. This instrument has been so often described in the public prints that we will not attempt to do it here, for the reason that it would be impossible without elaborate drawings and specifications. It is unnecessary to state that it differs in a fundamental way from other facsimile systems of telegraphy. Suffice it to say that as one writes his message in one city another pen in another city follows the transmitting-pen with perfect synchronism; it is as though a man were writing with a pen with two points widely separated, both moving at the same time and both making exactly the same motions. By this system a man may transact business with the same accuracy as by the United States mail, and with the same celerity as by the electric telegraph.
A broker may buy or sell with his own signature attached to the order, and do it as quickly as he could by any other method of telegraphing, and with absolute accuracy, secrecy and perfect identification.
In 1893, when this apparatus was first publicly exhibited, it operated by means of four wires between stations, and while the work it did was faultless, the use of four wires made it too expensive and too cumbersome for commercial purposes; so during all the years since then the endeavor has been to reduce the number of wires to two, when it would stand on an equality with the telephone in this respect. It is only lately that this improvement has been satisfactorily accomplished, and, for reasons above stated, no serious attempt has beenmade to introduce it as yet; but it has been used for a long enough time to demonstrate its practicability and commercial value. Companies have been organized both in Europe and America for the purpose of putting the telautograph into commercial use.
By means of a switch located in each subscriber's office the wires may be switched from a telephone to a telautograph, or vice versa, in a moment of time. By this arrangement a man may do all the preliminary work of a business transaction through the telephone, and when he is ready to put it into black and white switch in the telautograph and write it down. For ordinary exchange work this is undoubtedly the true way to use the telautograph, because one system of wires and one central-station system will answer for both modes of communication, and in this way an enormous saving can be made to the public. There is no question in the mind of any one who is familiar with the operation of both the telephone and telautograph but that some day they will both be used, either in the same or separate systems, as they each have distinctly separate fields of usefulness,—the telephone for desultory conversation, the telautograph for accurate business transactions. The question may arise in the minds of experts how the two systems can be worked in the same set ofcables, and this leads us to discuss the phenomena of induction.
Every one who has listened at a telephone has heard a jumble of noises more or less pronounced, which is the effect of the working of other wires in proximity to those of the telephone. If, when a Morse telegraph instrument is in operation on one of a number of wires strung on the same poles, we should insert a telephone in any one of the wires that were strung on the same poles or on another set of poles even across the street, we could hear the working of this Morse wire in the telephone, more or less pronounced, according to the distance the wire is from the Morse circuit. This phenomenon is the result of induction, caused by magnetic ether-waves that are set up whenever a circuit is broken and closed, as explained in Chapter VI.
The telephone is perhaps the most sensitive of all instruments, and will detect electrical disturbances that are too feeble to be felt on almost any other instrument, hence the telephone is preyed upon by every other system of electrical transmission, and for this reason has to adopt means of self-protection. It has been found that the surest way to prevent interference in the telephone from neighboring wires is to use what is called a metallic circuit—that is to say, instead of running a single wire from point to point and grounding at each end, asin ordinary telegraph systems, the telephone circuit is completed by using a second wire instead of the earth.
As a complete defense against the effects of induced currents the wires should be exactly alike as to cross-section (or size) and resistance. They should be insulated and laid together with a slight twist. This latter is to cause the two wires so twisted to average always the same distance from any contiguous wire.
One factor in determining the intensity of an induced current is the distance the wire in which it flows is from the source of induction. A telephone put in circuit at the end of the two wires that are thus laid together will be practically free from the effects of induced currents that are set up by the working of contiguous wires—for this reason: Whenever a current is induced in one of the slack-twisted wires it is induced in both alike; the two impulses being of the same polarity meet in the telephone, where they kill each other. In order to have a perfect result we must have perfect conditions, which are never attained absolutely, but nearly enough for all practical purposes.
In the early days of telephony great difficulty was experienced in using a single wire grounded at each end in the ordinary way, if it ran near other wires that were in active use.As time passed on and the electric light and electric railroad came into operation these difficulties were immensely increased, till now in large cities the telephone companies are fast being driven to the double-wire system, which will soon become universal for telephonic purposes the world over, except perhaps in a few country places where there is freedom from other systems of electrical transmission. To successfully work the telephone and telautograph through the same cables, these protective devices against induction must be very carefully provided and maintained.
Until within recent years it was never supposed that a sunbeam would ever laugh except in poetry. But the modern scientist has taken it out of the realm of poetry and put it into the prosy play of every-day life. The Radiophone, invented by A. G. Bell, is an instrument by which articulate or other sounds are transmitted through the medium of a ray of light. It has as yet no practical application and has never gone beyond the experimental stage, but as a bit of scientific information it is very interesting.
If we introduce into an electric circuit a piece of selenium, prepared in a certain way, its resistance as an electric conductor undergoes a radical change when a beam of sunlight is thrown upon it. For instance, a selenium cell, so called, that in the dark would measure 300 ohms resistance, would have only about 150 ohms when exposed to sunlight. This amount of variation in a short circuit of low resistance would produce a considerable changein the strength of a current passing through it from a battery of a given voltage.
If now we connect a selenium cell to one pole of a battery, and thence through a telephone and back to the other pole, we have completed an electric circuit, of which the selenium cell is a part, and any variation of resistance in this cell, if made suddenly, will be heard in the telephone. Let the diaphragm of a telephone transmitter have a very light, thin mirror on one side of it, and a beam of sunlight be thrown upon it and reflected from that on to the selenium cell, which may be some distance away. Then, if the diaphragm is thrown into vibration by an articulate word or other sound, the light-ray is also thrown into vibration, which causes a vibratory change of resistance in the selenium cell in sympathy with the light-vibrations; and this in turn throws the electric current into a sympathetic vibratory state which is heard in the telephone. So that if a person laughs or talks or sings to the diaphragm, the sunbeam laughs, talks and sings and tells its story to the electric current, which impresses itself upon the telephone as audible sounds—articulate or otherwise. Instead of the telephone, battery and selenium cell, a block of vulcanite or certain other substances may be used as a receiver; as a light-ray thrown into vibrationhas the power to produce sound or sympathetic vibration in certain substances.
Another curious application of the selenium cell has been attempted, but has scarcely gone beyond the domain of theory. This apparatus, if perfected, might be called a Telephote. It is an apparatus by which an illuminated picture at one end of a line of many wires is reproduced upon a screen at the other end. The light is not actually transmitted, but only its effects. Suppose a picture is laid off into small squares and there is a selenium cell corresponding to each square and for each selenium cell there is a wire that runs to a distant station in which circuit there is a battery. At the distant station there are little shutters, one for each wire, that are controlled by the electric current and so adjusted that when the cell at the transmitting-end is in the dark the shutter will be closed. Now if a strong light be thrown upon the picture at the transmitting-end, and each square of the picture reflects the light upon its corresponding selenium cell, the high lights of the picture will reflect stronger light than the shadows, and therefore the wires corresponding to the high-light squares will have a stronger current of electricity flowing through them, because the resistance of the circuit is less than the ones connected with the darker shadows. So that the degree of current-strength in the various wires will correspond to the intensity of light reflected by the different sections of the picture. The shutters are so adjusted that the amount of opening depends upon the strength of current. The shutters corresponding to the high lights of the picture will open the widest and throw the strongest light upon the screen, from a source of light that is placed behind the shutters. The shutters that open the least will be those that are operated upon by the shadows of the picture. Inasmuch as a picture thrown on a screen from a source of light is wholly made up of lights and shadows, the theory is that this apparatus perfectly constructed would transmit any picture to a distance, through telegraph-wires. It must not be understood that the rays of light are transmitted through the wires as sound-vibrations are. Light, per se, can be transmitted only through the luminiferous ether, as we have seen in the chapter on light in Volume II.
While we are talking about these curious methods of telegraphic transmission, I wish to refer to an apparatus constructed by the writer in 1874-5, for the purpose of receiving musical tones or compositions transmitted from a distance through a wire by electricity. (A cut of this apparatus is shown on page 875 of "Electricity and Electric Telegraph," by Prescott, issued in 1877.) It consists of a diskof metal rotated by a crank mounted on a suitable stand. The electric circuit passes through the disk to the hand of the operator in contact with it, thence running through the line-wire to the distant station. Now, if a tune is played at that station, upon an electrical key-board, as described in a previous chapter, and the disk rotated with the fingers in contact with it, the tune or other sounds will be reproduced at the ends of the fingers. After the telephone was invented and put into use I used this revolving disk as a receiver for speech as well as music, and by this means persons may carry on an oral conversation through the ends of their fingers. This apparatus has been confounded in the minds of some people with Edison's electromotograph. The phenomena of the electromotograph were produced by chemical effects, while that of the apparatus just described is electrostatic in its action. The electrostatic disk was made in the winter of 1873-4, while Edison's electrochemical discovery was made some time later.
Broadly speaking, "Wireless Telegraphy" is any method of transmitting intelligible signals to a distance without wires; and this includes the old Semaphore systems of visual signals, such as flags and long arms of wood by day, and lights by night; also the Heliograph (an apparatus for flashing sunlight), and Sound Signals, made either through the air or water. Electrical conduction, either through rarefied air or the earth, also comes under this heading.
The name "Wireless Telegraphy," however, is specifically applied to a system of signaling by means of ether-waves induced by electrical discharges of very high voltage. Ether-waves of a greater or less degree are always set up whenever there are sudden electrical disturbances, however slight. Ether-waves, electrically induced, are probably as old as the universe. When "there were thunders and lightnings" from the cloud that hovered over Mount Sinai in the time of Moses, ether-waves of great power were sent out through the campof Israel. But the people of those days had no "coherer" or telephone or any other means of converting these waves into visual or audible signals. Thousands of years had to elapse before the intellect of man could grasp the meaning of these natural phenomena sufficiently to harness them and make them subservient to his will.
Many people have been powerfully "shocked"—some even killed—by the impact of ether-waves set up by powerful discharges of lightning between the clouds and the earth—when they were not in the direct path of the lightning-stroke.
The history of Electro-Wireless Telegraphy, like that of all inventions, is one of successive stages, and all the work was not done by one man. The one who gets the most credit is usually the one who puts on the finishing touches and brings it out before the public. He may have done much toward its development or he may have done but little.
In the year 1842 Morse transmitted a battery current through the water of a canal eighty feet wide so as to affect a galvanometer on the opposite side from the battery. This was wireless telegraphy byconductionthrough water.
In 1835 Joseph Henry produced an effect on a galvanometer by ether-waves through a distance of twenty feet by an arrangement ofbatteries and circuits like that shown inFig. 1, Chapter VI. This was calledinduction, and is still so called when electrical effects are produced from one wire to another through the ether for short distances. All induction-coils and transformers (see Chapter XXIV) are operated by effects produced through the ether from the primary to the secondary coil—but through very short distances.
In 1880 Professor Trowbridge transmitted an electrical current through the earth for one mile so as to produce signals in a telephone. In 1881-2 Professor Dolbear used for a short distance (fifty feet) substantially the same arrangement as Marconi now uses, except that the former used a telephone as a receiver. He used an induction-coil having one end of the secondary wire connected with the earth, while the other was attached to a wire running up into the air. At the receiving-end a wire starting from the earth extended into the air, passing through a telephone, which acted as a receiver. In 1886 he used a kite to elevate the wire, through which electrical discharges of high voltage were made into the air to produce ether-waves—the receiver being 2000 feet away. Dolbear's experiments were public fourteen years ago, but at that time there was no interest in such matters, so that his work received little or no attention. In 1887 Dr. Hertz of Germany made some experiments inproducing and detecting ether-waves, and he did a great deal to awaken an interest in the subject, so that others began investigations that have led to its present use as a means of telegraphing to a distance of many miles.
In 1891 Professor Branly of Paris invented the coherer. In 1894 it was improved by Lodge and by him used as a detector of ether-waves. In 1896, ten years after Dolbear had used it with the kite at the transmitting-end and telephone at the receiving-end, Marconi, an Italian, substituted the coherer of Branly for the telephone of Dolbear. This coherer is constructed and operated as follows:
It consists of a glass tube, of comparatively small diameter, loosely filled with metal filings of a certain grade. This body of metal-dust is made a part of a local battery circuit in which is placed an ordinary electric bell or telegraphic sounder. The resistance of this body of filings is so great that current enough will not pass through it to ring the bell or actuate the sounder until an ether-wave strikes it and the wire attached to it, when the metal particles are made to cohere to such an extent that the conductivity of the mass is greatly increased; so that a current of sufficient volume will now pass through the bell-magnet to ring it. Before the next signal comes the filings must be made to de-cohere; and to accomplish this a little "tapper," that works automaticallybetween the signals, strikes the glass tube with a succession of light blows.
Briefly stated, the wireless system of Marconi, in its essentials, consists of a powerful induction-coil with one end of the secondary wire connected with the earth, while the other extends into the air a greater or less distance according to the distance it is desired to send signals. The greater the distance the higher the wire should extend into the air. At the receiving-end a wire of corresponding height is erected, also connected with the earth. In this wire—as a part of its circuit—is placed the coherer. In a local circuit that is connected to the upright wire in parallel with the coherer is placed a battery, a sounder, or a bell, that is rung when the filings cohere.
When an ether-wave is set up by a discharge of electricity into the air it strikes the perpendicular wire of the receiver, and that portion of the wave that strikes is converted into electricity, which is called an induced current. It is this current, as it discharges through the coherer to the earth, that causes the filings to unite so as to close the local circuit and operate the sounder. To send a message it is only necessary to make the discharges into the air, at the sending-end, correspond to the Morse alphabet.
While Marconi has done more than any other man to improve and popularize wirelesstelegraphy, history shows that he invented none of the essential elements so far as the system has been made public.
What he seems to have really done was to substitute the coherer of Branly and Lodge, with its adjuncts, for the telephone of Dolbear. There is no doubt but that Marconi has done much to improve and enlarge the capacity of the apparatus and to demonstrate to the world some of its possibilities. He has been an indefatigable worker and deserves great credit; but without the work of those who preceded him he could not have succeeded: the honors should be divided.
This system has been used at various times for reporting yacht-races, and between ships. It is said also to have been used to some extent in the South African War. There is much to be done yet, however, before it can be made entirely reliable for defensive work in time of war. As it is now, all an enemy would have to do to destroy its usefulness would be to set an ether-wave-producer to work automatically anywhere within the "sphere of influence" of the system—to speak diplomatically—when it would render unintelligible any message that should be sent. To make the system of the greatest value some sort of selective receiver must be invented that will select signals sent from a transmitter that is designed to work with it.
There is no doubt but that wireless telegraphy will some time play an important part in many spheres of usefulness.
There is another mode (already referred to) for transmitting signals electrically without wires through the earth instead of through the air, but in this case it is not through the medium of induction, but conduction. It has been explained in former chapters that earth-currents are constantly flowing from one point to another where the potentials are unequal. Sometimes these inequalities of potential are caused by heat and sometimes by electricity, as in the case of a thunder-storm. If a cloud is heavily charged with positive electricity, say, the earth underneath will have an equal charge of negative electricity. Let us illustrate it by the tides. As the moon passes over the ocean it attracts the water toward it and tends to pile up, as it were, at the nearest point between the earth and the moon. Suppose that (while the water is thus piled up at a point under the moon) we could suddenly suspend the attraction between the earth and the moon—the water would begin immediately to flow off by the force of gravitation until it had found a common level. Suppose in the place of the moon we have a cloud containing a static charge of positive electricity—it attracts a negative charge to a point on the earth nearest the cloud. If now a dischargetakes place between the earth and cloud the potential between the two will suddenly become equalized and the static charge that was accumulated in the earth is released and it dissipates in every direction, seeking an equilibrium, following the analogy of the water; the difference being that in one case the movement is very slow, while in the other it is as "quick as lightning."