The Story of Electricity

by John Munro

The inner star of twelve copper wires is the conductor, and the black circle round it is the gutta-percha or insulator which keeps the electricity from escaping into the water. The core in shallow water is protected from the bites of teredoes by a brass tape, and the envelope or armour consists of hemp and iron wire preserved from corrosion by a covering of tape and a compound of mineral pitch and sand.

The circuit of a submarine line is essentially the same as that of a land line, except that the earth connection is usually the iron sheathing of the cable in lieu of an earth-plate. On a cable, however, at least a long cable, the instruments for sending and receiving the messages are different from those employed on a land line. A cable is virtually a Leyden jar or condenser, and the signal currents in the wire induce opposite currents in the water or earth. As these charges hold each other the signals are retarded in their progress, and altered from sharp sudden jets to lagging undulations or waves, which tend to run together or coalesce. The result is that the separate signal currents which enter a long cable issue from it at the other end in one continuous current, with pulsations at every signal, that is to say, in a lapsing stream, like a jet of water flowing from a constricted spout. The receiving instrument must be sufficiently delicate to manifest every pulsation of the current. Its indicator, in fact, must respond to every rise and fall of the current, as a float rides on the ripples of a stream.

Such an instrument is the beautiful "mirror" galvanometer of Lord Kelvin, Ex-President of the Royal Society, which we illustrate in figure 52, where C is a coil of wire with a small magnetic needle suspended in its heart, and D is a steel magnet supported over it. The needle (M figure 53) is made of watch spring cemented to the back of a tiny mirror the size of a half-dime which is hung by a single fibre of floss silk inside an air cell or chamber with a glass lens G in front, and the coil C surrounds it. A ray of light from a lamp L (figure 52) falls on the mirror, and is reflected back to a scale S, on which it makes a bright spot. Now, when the coil C is connected between the end of the cable and the earth, the signal current passing through it causes the tiny magnet to swing from side to side, and the mirror moving with it throws the beam up and down the scale. The operator sitting by watches the spot of light as it flits and flickers like a fire-fly in the darkness, and spells out the mysterious message.

A condenser joined in the circuit between the cable and the receiver, or between the receiver and the earth, has the effect of sharpening the waves of the current, and consequently of the signals. The double-current key, which reverses the poles of the battery and allows the signal currents to be of one length, that is to say, all "dots," is employed to send the message.

Another receiving instrument employed on most of the longer cables is the siphon recorder of Lord Kelvin, shown in figure 54, which marks or writes the message on a slip of travelling paper. Essentially it is the inverse of the mirror instrument, and consists of a light coil of wire S suspended in the field between the poles of a strong magnet M. The coil is attached to a fine siphon (T5) filled with ink, and sometimes kept in vibration by an induction coil so as to shake the ink in fine drops upon a slip of moving paper. The coil is connected between the cable and the earth, and, as the signal current passes through, it swings to one side or the other, pulling the siphon with it. The ink, therefore, marks a wavy line on the paper, which is in fact a delineation of the rise and fall of the signal current and a record of the message. The dots in this case are represented by the waves above, and the "dashes" by the waves below the middle line, as may be seen in the following alphabet, which is a copy of one actually written by the recorder on a long submarine cable.

Owing to induction, the speed of signalling on long cables is much slower than on land lines of the same length, and only reaches from 25 to 45 words a minute on the Atlantic cables, or 30 to 50 words with an automatic sending-key; but this rate is practically doubled by employing the Muirhead duplex system of sending two messages, one from each end, at the same time.

The relation of the telegraph to the telephone is analogous to that of the lower animals and man. In a telegraph circuit, with its clicking key at one end and its chattering sounder at the other, we have, in fact, an apish forerunner of the exquisite telephone, with its mysterious microphone and oracular plate. Nevertheless, the telephone descended from the telegraph in a very indirect manner, if at all, and certainly not through the sounder. The first practical suggestion of an electric telephone was made by M. Charles Bourseul, a French telegraphist, in 1854, but to all appearance nothing came of it. In 1860, however, Philipp Reis, a German schoolmaster, constructed a rudimentary telephone, by which music and a few spoken words were sent. Finally, in 1876, Mr. Alexander Graham Bell, a Scotchman, residing in Canada, and subsequently in the United States, exhibited a capable speaking telephone of his invention at the Centennial Exhibition, Philadelphia.

Figure 56 represents an outside view and section of the Bell telephone as it is now made, where M is a bar magnet having a small bobbin or coil of fine insulated wire C girdling one pole. In front of this coil there is a circular plate of soft iron capable of vibrating like a diaphragm or the drum of the ear. A cover shaped like a mouthpiece O fixes the diaphragm all round, and the wires W W serve to connect the coil in the circuit.

The soft iron diaphragm is, of course, magnetised by the induction of the pole, and would be attracted bodily to the pole were it not fixed by the rim, so that only its middle is free to move. Now, when a person speaks into the mouthpiece the sonorous waves impinge on the diaphragm and make it vibrate in sympathy with them. Being magnetic, the movement of the diaphragm to and from the bobbin excites corresponding waves of electricity in the coil, after the famous experiment of Faraday (page 64). If this undulatory current is passed through the coil of a similar telephone at the far end of the line, it will, by a reverse action, set the diaphragm in vibration and reproduce the original sonorous waves. The result is, that when another person listens at the mouthpiece of the receiving telephone, he will hear a faithful imitation of the original speech.

The Bell telephone is virtually a small magneto-electric generator of electricity, and when two are joined in circuit we have a system for the transmission of energy. As the voice is the motive power, its talk, though distinct, is comparatively feeble, and further improvements were made before the telephone became as serviceable as it is now.