The system is a combination of telegraphic apparatus and telephone receiver, although in this case the latter instrument is not used in the generally understood manner. It is well known that the diaphragm of a telephone vibrates with the fluctuations of the current energizing the magnet beneath it. If the make and break of the magnetizing current be rapid, the vibrations being within the limits of the human ear, the diaphragm will produce an audible sound; but if the make and break be as slow as with ordinary Morse transmission, the diaphragm will be merely flexed and return to its original form without producing a sound. If, therefore, there be placed in the same circuit a regular telegraph relay and a special telephone, an operator may, by manipulating a key, operate the relay (and its sounder) without producing a sound in the telephone, as the makes and breaks of the key are far below the limit of audibility. But if through the same circuit, by means of another key suitably connected there is sent the rapid changes in current from an induction-coil, it will cause a series of loud clicks in the telephone, corresponding to the signals transmitted; but this current is too weak to affect the telegraph relay. It will be seen, therefore, that this method of duplexing is practiced, not by varying the strength or polarity, but by sending TWO KINDS OF CURRENT over the wire. Thus, two sets of Morse signals can be transmitted by two operators over one line at the same time without interfering with each other, and not only between terminal offices, but also between a terminal office and any intermediate office, or between two intermediate offices alone.

III

AUTOMATIC TELEGRAPHY

FROM the year 1848, when a Scotchman, Alexander Bain, first devised a scheme for rapid telegraphy by automatic methods, down to the beginning of the seventies, many other inventors had also applied themselves to the solution of this difficult problem, with only indifferent success.

"Cheap telegraphy" being the slogan of the time, Edison became arduously interested in the subject, and at the end of three years of hard work produced an entirely successful system, a public test of which was made on December 11, 1873 when about twelve thousand (12,000) words were transmitted over a single wire from Washington to New York. in twenty-two and one-half minutes. Edison's system was commercially exploited for several years by the Automatic Telegraph Company, as related in the preceding narrative.

As a premise to an explanation of the principles involved it should be noted that the transmission of telegraph messages by hand at a rate of fifty words per minute is considered a good average speed; hence, the availability of a telegraph line, as thus operated, is limited to this capacity except as it may be multiplied by two with the use of the duplex, or by four, with the quadruplex. Increased rapidity of transmission may, however, be accomplished by automatic methods, by means of which, through the employment of suitable devices, messages may be stamped in or upon a paper tape, transmitted through automatically acting instruments, and be received at distant points in visible characters, upon a similar tape, at a rate twenty or more times greater--a speed far beyond the possibilities of the human hand to transmit or the ear to receive.

In Edison's system of automatic telegraphy a paper tape was perforated with a series of round holes, so arranged and s.p.a.ced as to represent Morse characters, forming the words of the message to be transmitted.

This was done in a special machine of Edison's invention, called a perforator, consisting of a series of punches operated by a bank of keys--typewriter fashion. The paper tape pa.s.sed over a cylinder, and was kept in regular motion so as to receive the perforations in proper sequence.

The perforated tape was then placed in the transmitting instrument, the essential parts of which were a metallic drum and a projecting arm carrying two small wheels, which, by means of a spring, were maintained in constant pressure on the drum. The wheels and drum were electrically connected in the line over which the message was to be sent. current being supplied by batteries in the ordinary manner.

When the transmitting instrument was in operation, the perforated tape was pa.s.sed over the drum in continuous, progressive motion. Thus, the paper pa.s.sed between the drum and the two small wheels, and, as dry paper is a non-conductor, current was prevented from pa.s.sing until a perforation was reached. As the paper pa.s.sed along, the wheels dropped into the perforations, making momentary contacts with the drum beneath and causing momentary impulses of current to be transmitted over the line in the same way that they would be produced by the manipulation of the telegraph key, but with much greater rapidity. The perforations being so arranged as to regulate the length of the contact, the result would be the transmission of long and short impulses corresponding with the dots and dashes of the Morse alphabet.

The receiving instrument at the other end of the line was constructed upon much the same general lines as the transmitter, consisting of a metallic drum and reels for the paper tape. Instead of the two small contact wheels, however, a projecting arm carried an iron pin or stylus, so arranged that its point would normally impinge upon the periphery of the drum. The iron pin and the drum were respectively connected so as to be in circuit with the transmission line and batteries. As the principle involved in the receiving operation was electrochemical decomposition, the paper tape upon which the incoming message was to be received was moistened with a chemical solution readily decomposable by the electric current. This paper, while still in a damp condition, was pa.s.sed between the drum and stylus in continuous, progressive motion. When an electrical impulse came over the line from the transmitting end, current pa.s.sed through the moistened paper from the iron pin, causing chemical decomposition, by reason of which the iron would be attacked and would mark a line on the paper. Such a line would be long or short, according to the duration of the electric impulse. Inasmuch as a succession of such impulses coming over the line owed their origin to the perforations in the transmitting tape, it followed that the resulting marks upon the receiving tape would correspond thereto in their respective lengths.

Hence, the transmitted message was received on the tape in visible dots and dashes representing characters of the Morse alphabet.

The system will, perhaps, be better understood by reference to the following diagrammatic sketch of its general principles:

Some idea of the rapidity of automatic telegraphy may be obtained when we consider the fact that with the use of Edison's system in the early seventies it was common practice to transmit and receive from three to four thousand words a minute over a single line between New York and Philadelphia. This system was exploited through the use of a moderately paid clerical force.

In practice, there was employed such a number of perforating machines as the exigencies of business demanded. Each machine was operated by a clerk, who translated the message into telegraphic characters and prepared the transmitting tape by punching the necessary perforations therein. An expert clerk could perforate such a tape at the rate of fifty to sixty words per minute. At the receiving end the tape was taken by other clerks who translated the Morse characters into ordinary words, which were written on message blanks for delivery to persons for whom the messages were intended.

This latter operation--"copying." as it was called--was not consistent with truly economical business practice. Edison therefore undertook the task of devising an improved system whereby the message when received would not require translation and rewriting, but would automatically appear on the tape in plain letters and words, ready for instant delivery.

The result was his automatic Roman letter system, the basis for which included the above-named general principles of perforated transmission tape and electrochemical decomposition. Instead of punching Morse characters in the transmission tape however, it was perforated with a series of small round holes forming Roman letters. The verticals of these letters were originally five holes high. The transmitting instrument had five small wheels or rollers, instead of two, for making contacts through the perforations and causing short electric impulses to pa.s.s over the lines. At first five lines were used to carry these impulses to the receiving instrument, where there were five iron pins impinging on the drum. By means of these pins the chemically prepared tape was marked with dots corresponding to the impulses as received, leaving upon it a legible record of the letters and words transmitted.

For purposes of economy in investment and maintenance, Edison devised subsequently a plan by which the number of conducting lines was reduced to two, instead of five. The verticals of the letters were perforated only four holes high, and the four rollers were arranged in pairs, one pair being slightly in advance of the other. There were, of course, only four pins at the receiving instrument. Two were of iron and two of tellurium, it being the gist of Edison's plan to effect the marking of the chemical paper by one metal with a positive current, and by the other metal with a negative current. In the following diagram, which shows the theory of this arrangement, it will be seen that both the transmitting rollers and the receiving pins are arranged in pairs, one pair in each case being slightly in advance of the other. Of these receiving pins, one pair--1 and 3--are of iron, and the other pair--2 and 4--of tellurium. Pins 1-2 and 3-4 are electrically connected together in other pairs, and then each of these pairs is connected with one of the main lines that run respectively to the middle of two groups of batteries at the transmitting end. The terminals of these groups of batteries are connected respectively to the four rollers which impinge upon the transmitting drum, the negatives being connected to 5 and 7, and the positives to 6 and 8, as denoted by the letters N and P. The transmitting and receiving drums are respectively connected to earth.

In operation the perforated tape is placed on the transmission drum, and the chemically prepared tape on the receiving drum. As the perforated tape pa.s.ses over the transmission drum the advanced rollers 6 or 8 first close the circuit through the perforations, and a positive current pa.s.ses from the batteries through the drum and down to the ground; thence through the earth at the receiving end up to the other drum and back to the batteries via the tellurium pins 2 or 4 and the line wire.

With this positive current the tellurium pins make marks upon the paper tape, but the iron pins make no mark. In the merest fraction of a second, as the perforated paper continues to pa.s.s over the transmission drum, the rollers 5 or 7 close the circuit through other perforations and t e current pa.s.ses in the opposite direction, over the line wire, through pins 1 or 3, and returns through the earth. In this case the iron pins mark the paper tape, but the tellurium pins make no mark. It will be obvious, therefore, that as the rollers are set so as to allow of currents of opposite polarity to be alternately and rapidly sent by means of the perforations, the marks upon the tape at the receiving station will occupy their proper relative positions, and the aggregate result will be letters corresponding to those perforated in the transmission tape.

Edison subsequently made still further improvements in this direction, by which he reduced the number of conducting wires to one, but the principles involved were a.n.a.logous to the one just described.

This Roman letter system was in use for several years on lines between New York, Philadelphia, and Washington, and was so efficient that a speed of three thousand words a minute was attained on the line between the two first-named cities.

Inasmuch as there were several proposed systems of rapid automatic telegraphy in existence at the time Edison entered the field, but none of them in practical commercial use, it becomes a matter of interest to inquire wherein they were deficient, and what const.i.tuted the elements of Edison's success.

The chief difficulties in the transmission of Morse characters had been two in number, the most serious of which was that on the receiving tape the characters would be prolonged and run into one another, forming a draggled line and thus rendering the message unintelligible. This arose from the fact that, on account of the rapid succession of the electric impulses, there was not sufficient time between them for the electric action to cease entirely. Consequently the line could not clear itself, and became surcharged, as it were; the effect being an attenuated prolongation of each impulse as manifested in a weaker continuation of the mark on the tape, thus making the whole message indistinct. These secondary marks were called "tailings."

For many years electricians had tried in vain to overcome this difficulty. Edison devoted a great deal of thought and energy to the question, in the course of which he experimented through one hundred and twenty consecutive nights, in the year 1873, on the line between New York and Washington. His solution of the problem was simple but effectual. It involved the principle of inductive compensation. In a shunt circuit with the receiving instrument he introduced electromagnets. The pulsations of current pa.s.sed through the helices of these magnets, producing an augmented marking effect upon the receiving tape, but upon the breaking of the current, the magnet, in discharging itself of the induced magnetism, would set up momentarily a counter-current of opposite polarity. This neutralized the "tailing"

effect by clearing the line between pulsations, thus allowing the telegraphic characters to be clearly and distinctly outlined upon the tape. Further elaboration of this method was made later by the addition of rheostats, condensers, and local opposition batteries on long lines.

The other difficulty above referred to was one that had also occupied considerable thought and attention of many workers in the field, and related to the perforating of the dash in the transmission tape. It involved mechanical complications that seemed to be insurmountable, and up to the time Edison invented his perforating machine no really good method was available. He abandoned the attempt to cut dashes as such, in the paper tape, but instead punched three round holes so arranged as to form a triangle. A concrete example is presented in the ill.u.s.tration below, which shows a piece of tape with perforations representing the word "same."

The philosophy of this will be at once perceived when it is remembered that the two little wheels running upon the drum of the transmitting instrument were situated side by side, corresponding in distance to the two rows of holes. When a triangle of three holes, intended to form the dash, reached the wheels, one of them dropped into a lower hole. Before it could get out, the other wheel dropped into the hole at the apex of the triangle, thus continuing the connection, which was still further prolonged by the first wheel dropping into the third hole. Thus, an extended contact was made, which, by transmitting a long impulse, resulted in the marking of a dash upon the receiving tape.

This method was in successful commercial use for some time in the early seventies, giving a speed of from three to four thousand words a minute over a single line, but later on was superseded by Edison's Roman letter system, above referred to.

The subject of automatic telegraphy received a vast amount of attention from inventors at the time it was in vogue. None was more earnest or indefatigable than Edison, who, during the progress of his investigations, took out thirty-eight patents on various inventions relating thereto, some of them covering chemical solutions for the receiving paper. This of itself was a subject of much importance and a vast amount of research and labor was expended upon it. In the laboratory note-books there are recorded thousands of experiments showing that Edison's investigations not only included an enormous number of chemical salts and compounds, but also an exhaustive variety of plants, flowers, roots, herbs, and barks.

It seems inexplicable at first view that a system of telegraphy sufficiently rapid and economical to be practically available for important business correspondence should have fallen into disuse. This, however, is made clear--so far as concerns Edison's invention at any rate--in Chapter VIII of the preceding narrative.

IV. WIRELESS TELEGRAPHY

ALTHOUGH Mr. Edison has taken no active part in the development of the more modern wireless telegraphy, and his name has not occurred in connection therewith, the underlying phenomena had been noted by him many years in advance of the art, as will presently be explained. The authors believe that this explanation will reveal a status of Edison in relation to the subject that has thus far been unknown to the public.

While the term "wireless telegraphy," as now applied to the modern method of electrical communication between distant points without intervening conductors, is self-explanatory, it was also applicable, strictly speaking, to the previous art of telegraphing to and from moving trains, and between points not greatly remote from each other, and not connected together with wires.

The latter system (described in Chapter XXIII and in a succeeding article of this Appendix) was based upon the phenomena of electromagnetic or electrostatic induction between conductors separated by more or less s.p.a.ce, whereby electric impulses of relatively low potential and low frequency set up in. one conductor were transmitted inductively across the air to another conductor, and there received through the medium of appropriate instruments connected therewith.

As distinguished from this system, however, modern wireless telegraphy--so called--has its basis in the utilization of electric or ether waves in free s.p.a.ce, such waves being set up by electric oscillations, or surgings, of comparatively high potential and high frequency, produced by the operation of suitable electrical apparatus.

Broadly speaking, these oscillations arise from disruptive discharges of an induction coil, or other form of oscillator, across an air-gap, and their character is controlled by the manipulation of a special type of circuit-breaking key, by means of which long and short discharges are produced. The electric or etheric waves thereby set up are detected and received by another special form of apparatus more or less distant, without any intervening wires or conductors.

In November, 1875, Edison, while experimenting in his Newark laboratory, discovered a new manifestation of electricity through mysterious sparks which could be produced under conditions unknown up to that time.

Recognizing at once the absolutely unique character of the phenomena, he continued his investigations enthusiastically over two mouths, finally arriving at a correct conclusion as to the oscillatory nature of the hitherto unknown manifestations. Strange to say, however, the true import and practical applicability of these phenomena did not occur to his mind. Indeed, it was not until more than TWELVE YEARS AFTERWARD, in 1887, upon the publication of the notable work of Prof. H. Hertz proving the existence of electric waves in free s.p.a.ce, that Edison realized the fact that the fundamental principle of aerial telegraphy had been within his grasp in the winter of 1875; for although the work of Hertz was more profound and mathematical than that of Edison, the principle involved and the phenomena observed were practically identical--in fact, it may be remarked that some of the methods and experimental apparatus were quite similar, especially the "dark box" with micrometer adjustment, used by both in observing the spark. [25]

[Footnote 25: During the period in which Edison exhibited his lighting system at the Paris Exposition in 1881, his representative, Mr. Charles Batchelor, repeated Edison's remarkable experiments of the winter of 1875 for the benefit of a great number of European savants, using with other apparatus the original "dark box" with micrometer adjustment.]

There is not the slightest intention on the part of the authors to detract in the least degree from the brilliant work of Hertz, but, on the contrary, to ascribe to him the honor that is his due in having given mathematical direction and certainty to so important a discovery.

The adaptation of the principles thus elucidated and the subsequent development of the present wonderful art by Marconi, Branly, Lodge, Slaby, and others are now too well known to call for further remark at this place.

Strange to say, that although Edison's early experiments in "etheric force" called forth extensive comment and discussion in the public prints of the period, they seemed to have been generally overlooked when the work of Hertz was published. At a meeting of the Inst.i.tution of Electrical Engineers, held in London on May 16, 1889, at which there was a discussion on the celebrated paper of Prof. (Sir) Oliver Lodge on "Lightning Conductors," however; the chairman, Sir William Thomson (Lord Kelvin), made the following remarks:

"We all know how Faraday made himself a cage six feet in diameter, hung it up in mid-air in the theatre of the Royal Inst.i.tution, went into it, and, as he said, lived in it and made experiments. It was a cage with tin-foil hanging all round it; it was not a complete metallic enclosing sh.e.l.l. Faraday had a powerful machine working in the neighborhood, giving all varieties of gradual working-up and discharges by 'impulsive rush'; and whether it was a sudden discharge of ordinary insulated conductors, or of Leyden jars in the neighborhood outside the cage, or electrification and discharge of the cage itself, he saw no effects on his most delicate gold-leaf electroscopes in the interior. His attention was not directed to look for Hertz sparks, or probably he might have found them in the interior. Edison seems to have noticed something of the kind in what he called the etheric force. His name 'etheric' may, thirteen years ago, have seemed to many people absurd. But now we are all beginning to call these inductive phenomena 'etheric.'"

With these preliminary observations, let us now glance briefly at Edison's laboratory experiments, of which mention has been made.

Oh the first manifestation of the unusual phenomena in November, 1875, Edison's keenness of perception led him at once to believe that he had discovered a new force. Indeed, the earliest entry of this discovery in the laboratory note-book bore that caption. After a few days of further experiment and observation, however, he changed it to "Etheric Force,"

and the further records thereof (all in Mr. Batchelor's handwriting) were under that heading.

The publication of Edison's discovery created considerable attention at the time, calling forth a storm of general ridicule and incredulity.

But a few scientific men of the period, whose experimental methods were careful and exact, corroborated his deductions after obtaining similar phenomena by repeating his experiments with intelligent precision. Among these was the late Dr. George M. Beard, a noted physicist, who entered enthusiastically into the investigation, and, in addition to a great deal of independent experiment, spent much time with Edison at his laboratory. Doctor Beard wrote a treatise of some length on the subject, in which he concurred with Edison's deduction that the phenomena were the manifestation of oscillations, or rapidly reversing waves of electricity, which did not respond to the usual tests. Edison had observed the tendency of this force to diffuse itself in various directions through the air and through matter, hence the name "Etheric"

that he had provisionally applied to it.

Edison's laboratory notes on this striking investigation are fascinating and voluminous, but cannot be reproduced in full for lack of s.p.a.ce.

In view of the later practical application of the principles involved, however, the reader will probably be interested in perusing a few extracts therefrom as ill.u.s.trated by facsimiles of the original sketches from the laboratory note-book.