Benjamin Franklin combated the hypothesis of Dufay, and suggested that there is only one electrie fluid, and that when a body is positively electrified it has an excess of the fluid, and when negatively electrified, a deficiency of it. No electrician nowadays believes that electricity is a fluid at all, but the old language is still retained for the convenience of reference to former works on the subject.

The mode of generating electricity by friction has been greatly improved upon, and the simple experiment of rubbing a glass rod by hand is in striking contrast to the elaborate plate-glass machines now employed. One of these in the recent Paris Electrical Exhibition took four men to work it, and yielded sparks as thick as a wheat straw, and twenty-three inches long. Newton was the first to employ a globe of glass rubbed by the hands, and Otto Von Guericke, the illustrious burgomaster of Magdeburg, used a ball of sulphur. Soon, however, the practice came in of employing cylinders or discs of glass rubbed by silk cushions; and the production of the electricity was increased by covering the silk with an amalgam of two parts of zinc, one of tin, and six of mercury, made into a paste with lard or butter.

The ordinary plate-glass machine is illustrated in Fig. 2, where T is a large circular plate of glass, mounted on an axle, which can be turned by the winch handle w. The rubbers rr rr are arranged in pairs at the top and bottom of the plate, which turns between each pair. The rubbers are horsehair cushions covered with silk, which has been coated with the paste referred to, and as the sides of the plate pass between them the friction generates positive electricity

on the glass. Flaps of oiled silk s s are attached to the rubbers to keep the electricity from discharging itself into the air, and two combs of brass points p p are presented to the sides

of the glass to tap the electricity and collect it on the "prime conductor." This conductor is simply a frame of brass tubing with rounded knobs, from which the positive electricity in the form of a spark may be drawn off. The negative electricity of the rubbers is also led away for use in some machines.

Machines giving

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FIG. 2.

have been inThe latest of

powerful sparks by inductive action vented by Holtz, Voss, and others. these, and the most remarkable of all, is the Wimshurst Influence machine, which we illustrate in Fig. 3. It consists of two circular glass plates c c, mounted on a common axle and about inch apart. They are rotated in opposite directions by turning the handle shown. Both discs are varnished, and on the outer surface of each are cemented twelve radial slips of thin brass s s', set round the border of each disc at equal angles. These play a part similar to the "carriers " in Sir W. Thomson's "mouse-mill." The two strips situated on the same diameter of each disc are twice in each revolution put in metallic connection by the fine wire brushes B B attached to the ends of the

curved brass rods. Collecting combs of brass with fine points P P grazing the glass accumulate the electricity generated on the positive and negative conductors, from which brass arcs curve upwards and end in balls D which form the sparking points. The best effect is got from the machine when the connecting bars B are at

an angle of 45° to the line of the collecting combs P, and 90° from each other. Sparks of four or five inches can be obtained from a duplex machine of this kind having plates fifteen inches in diameter, provided a small condenser, say a pint size Leyden jar, is connected to each collector. So rapid is the generation of the electricity

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.

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