Edison, His Life and Inventions [351]
We quote again from Mr. Hammer, who says: "The freight- locomotive had single reduction gears, as is the modern practice, but the power was applied through a friction-clutch The passenger-locomotive was very speedy, and ninety passengers have been carried at a time by it; the freight- locomotive was not so fast, but could pull heavy trains at a good speed. Many thousand people were carried on this road during 1882." The general appearance of Edison's electric locomotive of 1882 is shown in the illustration opposite page 462 of the preceding narrative. In the picture Mr. Edison may be seen in the cab, and Mr. Insull on the front platform of the passenger-car.
XIV
TRAIN TELEGRAPHY
WHILE the one-time art of telegraphing to and from moving trains was essentially a wireless system, and allied in some of its principles to the art of modern wireless telegraphy through space, the two systems cannot, strictly speaking be regarded as identical, as the practice of the former was based entirely on the phenomenon of induction.
Briefly described in outline, the train telegraph system consisted of an induction circuit obtained by laying strips of metal along the top or roof of a railway-car, and the installation of a special telegraph line running parallel with the track and strung on poles of only medium height. The train, and also each signalling station, was equipped with regulation telegraph apparatus, such as battery, key, relay, and sounder, together with induction-coil and condenser. In addition, there was a special transmitting device in the shape of a musical reed, or "buzzer." In practice, this buzzer was continuously operated at a speed of about five hundred vibrations per second by an auxiliary battery. Its vibrations were broken by means of a telegraph key into long and short periods, representing Morse characters, which were transmitted inductively from the train circuit to the pole line or vice versa, and received by the operator at the other end through a high-resistance telephone receiver inserted in the secondary circuit of the induction-coil.
The accompanying diagrammatic sketch of a simple form of the system, as installed on a car, will probably serve to make this more clear.
An insulated wire runs from the metallic layers on the roof of the car to switch S, which is shown open in the sketch. When a message is to be received on the car from a station more or less remote, the switch is thrown to the left to con- nect with a wire running to the telephone receiver, T. The other wire from this receiver is run down to one of the axles and there permanently connected, thus making a ground. The operator puts the receiver to his ear and listens for the message, which the telephone renders audible in the Morse characters.
If a message is to be transmitted from the car to a receiving station, near or distant, the switch, S, is thrown to the other side, thus connecting with a wire leading to one end of the secondary of induction-coil C. The other end of the secondary is connected with the grounding wire. The primary of the induction-coil is connected as shown, one end going to key K and the other to the buzzer circuit. The other side of the key is connected to the transmitting battery, while the opposite pole of this battery is connected in the buzzer circuit. The buzzer, R, is maintained in rapid vibration by its independent auxiliary battery, B<1S>.
When the key is pressed down the circuit is closed, and current from the transmitting battery, B, passes through primary of the coil, C, and induces a current of greatly increased potential in the secondary. The current as it passes into the primary, being broken up into short impulses by the tremendously rapid vibrations of the buzzer, induces similarly rapid waves of high potential in the secondary, and these in turn pass to the roof and thence through the intervening air by induction to the telegraph wire. By a continued lifting and depression of the key in the regular manner, these waves are broken up into long and short periods, and are thus transmitted