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that such a spark may be produced at every two and a half turns of the handle.

Electricity can also be generated by the friction of pure water globules against wood. This fact was discovered in 1840 by Mr. (now Sir William) Armstrong, whose apparatus for showing it consists of a high-pressure boiler, from which the steam escapes by a series of fine pipes lined with wood, and blows against a metal comb connected with a conductor. The friction against the wood charges the steam with positive electricity, which is delivered up to the teeth of the comb and the conductor, as in the ordinary electric machine. In this case also the nature of the touching surfaces is very important, for if turpentine be mixed with the water the steam becomes negatively charged.

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The use of the term "conductor leads us to the most important property of electricity, that is to say, its power of travelling through certain substances. In 1721 Stephen Gray, a pensioner of the Charterhouse, succeeded in transmitting the electricity from a glass machine along a wire for a considerable distance. The wire was suspended by hanks of silk thread, for Gray had made the discovery that some bodies-such as the metals, carbon, water-conducted electricity away, while others-such as silk, glass, resin, air-did not. The former are called conductors, the latter nonconductors, or more commonly, insulators. Silver is the best conductor of all known metals, and copper comes next, hence its use in making electrical instruments and the conducting wire of telegraph cables; while silk, india-rubber, gutta-percha, and paraffin wax, being non-conductors, are generally used to cover

the copper wire so as to isolate or confine the electricity upon it.

Stephen Gray's line was the germ of the electric telegraph, and soon there were plans invented for sending signals from one place to another by means of electricity conveyed along an insulated wire. Benjamin Franklin proposed to send visible signals along a conductor by electric sparks which would ignite alcohol at the farther end, and in 1748 he actually set fire to spirits by an electric current sent through a wire across the Schuylkill River. Moreover, in the "Scots Magazine" for 1753, there is a letter signed by one "C. M.," supposed to be a Mr. Charles Marshall, of Renfrew, in which there is a full description of a practical telegraph. His plan was to have wires equal in number to the letters of the alphabet stretched on poles between the two places which wished to correspond with one another. They were to be insulated from the poles by glass and jeweller's cement; an electric machine was to supply the electricity which traversed them, and light printed characters were to be visibly attracted by the electrified ends of the wires. "Suppose," said the writer, "I am to pronounce the word Sir. With a piece of glass, or any other electric (non-conductor) per se, I strike the wire S so as to bring it in contact with the barrel of the electric machine generating the electricity; then i, then r, all in the same way, and my correspondent almost in the same instant observes these several characters rise in order to the electrified ball at his end of the wire."

This and other plans of signalling by means of frictional electricity were, however, tried only to be abandoned. There was great difficulty in insulating

the spark on the wire, owing to its tendency to escape from fine points into the air, or to flow into the ground through the wood or other substances used as insulators. It was not until a more docile kind of electricity was discovered in the electric current produced by the voltaic battery that the electric telegraph as it exists now became practicable. These early experiments with frictional electricity, however, prepared the way for the later telegraph, by leading up to general laws and suggesting ideas. Moreover, they gave us the lightning-rod, which has proved invaluable in saving life and property from the thunderbolt both on land and sea. When Franklin, in the June of 1752, sent his kite up to the clouds and drew sparks from the end of the wetted string, which in this case served to conduct the atmospheric electricity, he demonstrated the identity of the dreaded lightningflash with the tiny spark from an electric machine. And knowing, as he did, that electricity can be conducted by a metal wire, and that it tends to discharge itself by points, it was an easy matter for him to devise a lightning-rod of copper with one end buried in the ground and the other rearing a fine point above the building to be protected. With this arrangement the electricity of the cloud would discharge itself into the wire by its fine point, and thereby find a harmless path into the ground.

CHAPTER II.

BATTERIES.

IN the year 1800 the celebrated Volta, Professor of Physics at Pavia, invented the chemical generator of electricity which is now known as the voltaic cell, and thereby inaugurated the remarkable development of electrical science which characterises the present century. Volta took plates of two different metals, copper and zinc, and separated them by a layer of acidulated water. On joining the two plates by a wire he found a current of electricity in the wire, and this current became stronger in proportion to the number of pairs of plates which he employed. Each pair of dissimilar metals with its connecting layer of liquid formed a simple element, and a number of these elements in combination formed a battery. The elementary voltaic cell, as shown in Fig. 4, consists of the plate of zinc z and a plate of copper c, plunged in a vessel of water tinctured with sulphuric acid and connected outside the cell by a wire w w. When the wire w w is disconnected or broken, no current of electricity is obtained; but when this wire is connected, the voltaic "circuit" is completed, and

FIG. 4.

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the current flows in the wire. Chemical action then goes on in the cell, the zinc is oxidised by the oxygen of the water, just as coal is burned in a fire, and the chemical action keeps up the supply of electricity just as the burning coal keeps up the supply of heat. No matter how long the connecting wire is, the current will flow through it, provided it is entire, and hence it is that we have telegraph lines two or three thousand miles long.

The water in this combination supplies the oxygen to burn the zinc, and sulphuric acid is merely added to reduce the resistance of the water to the passage of the electricity through it, for the current starts from the zinc plate where the chemical action takes place, and flows to the copper plate within the cell, then through the wire back to the zinc plate, thus completing its round. The hydrogen set free by the breaking up of the water to yield oxygen to the zinc is given off in the form of gas.

This elementary cell has, of course, been greatly improved upon, and there are now hundreds of different combinations of solids and liquids serving as voltaic batteries. The chief of these are the Daniell battery, the Leclanché battery, the Bunsen, Grove, and Smee batteries. The Daniell battery is the elementary cell we have described, with crystals of blue vitriol, or sulphate of copper, added round the copper plate. This salt is decomposed by the free hydrogen into pure copper and sulphuric acid, so that the hydrogen, instead of being uselessly given off to the air, helps to provide sulphuric acid to reduce the internal resistance of the cell. Moreover, as all the hydrogen is not discharged in the elementary cell, but part clings to the

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