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THE EDISON SYSTEM OF RAILWAY TELEGRAPHY.
Scientific American—February 20, 1886

 

The announcement made last summer that Mr. Thomas A. Edison was working out the details of a system of inductive telegraphy for sending and receiving messages on moving trains prepared the public for taking a very lively interest in the week's practical trial of the system, recently made on the Staten Island Railway. The necessary apparatus was applied to a car on one of the regular afternoon trains running from Clifton to Tottenville. The experiments were made personally interesting by having each member of the party leave a written message, sealed and directed to himself, with the operator at Clifton. During the trip, these messages were received on board the moving train, and each writer had the satisfaction of having his words correctly returned to him.

The main feature of the system, that of using the ordinary telegraph wires strung on the poles along the track, in place of a specially laid wire, as in the Phelps system, was invented and patented by Mr. William Wiley Smith in the fall of 1881, and he and Mr. E. T. Gilliland have been associated with Mr. Edison in the development of this idea.

The apparatus on board the train is all attached to a compact operator's desk, occupying no more room than an ordinary car seat. This desk may be placed in any part of the car desired, arid may be moved from one car to another in a few minutes' time. The apparatus consists of an ordinary Morse key, phonetic receivers, an electro-magnet, a vibrating reed, an induction coil, and a battery. Similar apparatus is in use at the fixed stations. With the assistance of our diagrammatic illustrations, the reader will be able to understand the disposition of the instruments, and follow the current on its journey from the key on the train to the receiver at the fixed station, or vice versa. The car roofs are covered with tin and connected electrically by copper wire. During the experimental run at Staten Island, four cars were used. As the induction takes place between the telegraph wires and the tin roof, it is desirable to have as large a metallic surface as possible. Under favorable conditions, one roof will suffice, but it is better to have several. An insulated wire runs from the roof of the telegraphing car to a switch, S, at the operator's desk. This is shown open in Fig. 1. When a message is to be received on the car, the switch is turned to connect with a wire running to the phonetic receiver, T, and thence to the, ground. The receiver may be either an ordinary telephone, or a pair may be used and held to the ears, somewhat after the manner of ear muffs. After coming from receiver, the wire is carried under the car and connected with a strip of copper, which is pressed against a copper cylinder on one of the axles by means of a spring, thus giving a ground connection by the, axle and wheel.

When, however, a message is to be transmitted from the car, the switch is connected with a wire leading to one end of a secondary coil at C, the other end of which is connected with the ground wire just described. Inside of this secondary coil, and separated from it by a layer of paper, is the primary coil, which is within the short electrical circuit represented by the diagram. A ten cell Fuller battery is placed underneath the desk. One pole of the battery is connected with the Morse key, K, which in turn is connected with one end of the primary coil as shown. The other pole of the battery is connected with a metallic reed, R, which is made to vibrate 500 times a second by means of a small independent battery. These vibrations produce a sharp, clear musical note, which is very audible at short distances from the operator's desk. The free end of the reed at each vibration strikes against a metallic button, connected by a wire with the other end of the primary coil. This being the arrangement of the apparatus on the train, we will suppose that the message is actually being transmitted. The Morse alphabet of dash and dot is employed, and consequently any operator can work the system. When the key is pressed down to form a letter, the short circuit is closed and the current passes through the primary coil. But the vibrating read breaks the current each second into five hundred waves, and these electrical waves induce corresponding ones in the secondary coil. The function of this induction coil is to transform the intermittent current into one of high electrical tension. From the coil the waves pass to the roof, and by a sharp, quick discharge traverse the intervening air and reach the telegraph wires. It will be noticed that the word "discharge" is employed in spite of the fact that the action between the roof and wires is nevertheless, in strict electrical parlance, one of induction. This constitutes the point of the invention.

Mr. Edison believes that he has made a new discovery in physics. He finds that bodies hitherto considered nonconductors, such as air, are really so only after an appreciable period of time. At the first instant of discharge, the air offers no resistance to the passage of a current, but becomes almost immediately polarized, and the communication becomes permanently interrupted. The idea, therefore, in these very short waves of high tension is to permit them to cross to the wires before the air has time to offer any opposition. A sufficient period, however intervenes between them to permit the air to return to its normal condition, and consequently allow the succeeding waves to pass. Mr. Edison illustrated this point to the writer by drawing a series of dots about an inch apart. The dots represent the waves and the spaces the intervening air. The polarization of the air attendant upon the passage of the waves is neutralized before the arrival of the wave following. The series of waves, having been communicated to the telegraph wires, are transmitted to every station on the line and to every train having suitable apparatus. If the key were held down continuously, simply a musical note, corresponding to that produced by the vibrating reed, would be heard in all the receivers. It is the breaking up of this note into dots and dashes, by means of the key, that transmits the telegram.

Though the apparatus at the fixed stations is similar to that on board the trains, the manner of throwing the waves on or off the telegraph wires is naturally different. Two arrangements are possible. In the one shown in Fig. 2, condensers are used. These are simply a series of circular metallic plates, equal in number to the number of telegraph wires used. One plate is connected with each wire, and is brought opposite, but not touching, a similar plate connected with the switch, S, in the station. The electrical waves pass to the condensers, spring across the intervening space to those opposite, and so pass on to the receivers.

In the second arrangement, shown in Fig. 3, no condensers are used, but the telegraph wires themselves are formed into a primary coil, and the wires from the office switch form the secondary coil. This is shown at C. In this case no further induction coil is needed in the office. The number of wires necessary for the transmission of the waves will depend upon circumstances. In case the sound becomes too faint for convenient translation into words, a greater number of wires should be employed. At Staten Island four were used and gave very satisfactory results. The distance from the car roof to the wires was from 15 to 20 feet. In experiments made at Menlo Park, Mr. Edison succeeded in transmitting a message through the air over a distance of 580 feet. It will be observed that the circuit connecting the operator's key and the receiver is three times broken by the air, once at the induction coil, once at the roof, and again at the fixed station, either at the condensers or at the line coil.

 

As one of the chief merits of this system is its inexpensiveness, it may be of interest to state just what the cost is. The apparatus on board the train and that at the fixed stations cost about $50 each. The annual royalty for the use of the system varies from $7 per mile on roads of five to. six thousand miles to $15 per mile on roads of a hundred miles or less. The president of the company, Dr. Crowell, informs us that it is about to be introduced on the line of the Chicago, Milwaukee, and St. Paul Railway. Of its usefulness in averting accidents, by keeping each train informed of the whereabouts of the one immediately ahead or following it, in intercepting criminals, and in promoting general social and commercial intercourse, it will be unnecessary to speak.

 

We are indebted to Frank Leslie's Illustrated Newspaper for the use of the large cut.


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