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terms as possible. For the sake of greater simplicity, we begin by recalling the elementary facts.

Electricity, as every schoolboy is taught, takes its name from the Greek word elektron, signifying "amber," because the famous Greek philosopher, Thales of Miletus, found out that when a piece of amber was rubbed it acquired the power of attracting very light bodies, such as grains of dust or bits of straw. Thales lived six hundred years before Christ, and for more than two thousand years little else was known about the mysterious agency which works so many wonders in our own day. After the revival of learning, however, it was discovered by Dr. Gilbert, of Colchester, in the year 1600, that glass, sulphur, resin, and many other substances behaved like amber when rubbed, and the true science of electricity then began.

The kind of electricity generated in this way is called "frictional electricity," and its existence is generally demonstrated in the following way. A dry glass tube is taken and vigorously rubbed with a silk handkerchief, and then brought near to a small pith ball suspended by a silk thread from the arm of a bracket which has a glass stem. The electricity excited on the rod by the friction of the silk will attract the pith ball (as shown in Fig. 1, where & is the rod and p the ball). Almost as soon, however, as the ball touches the rod it will fly off again and take up the position p' with respect to the rod G'. The fact is, that in touching the rod the ball pilfers

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FIG. 1.

some of its electricity, and is repelled. If, however, a rod of sealing-wax or resin is now rubbed with the silk and put in the place of the glass rod, the electrified ball will be attracted to the wax. The explanation of this preference is that the rubbing has charged the sealing-wax with an opposite kind of electricity to that on the glass and also on the ball. Hence we have the law that like kinds of electricity repel each other, and unlike kinds attract.

This experiment and many others of the same sort led the celebrated M. Dufay to imagine that there were two opposite kinds of electric fluid pervading all bodies. Having an attraction for each other, he held that they tended to mix and neutralise each other, thereby producing a state of electric quiet in the body. Rubbing separated the two fluids from each other and rendered them appreciable to our tests. It could be shown that when a body is electrified by rubbing, an equal quantity of the opposite kind of electricity was always excited on the rubber, just as if the act of rubbing merely divorced the two different fluids from each other. The electricity produced on glass by rubbing with silk he called the positive, or vitreous, fluid, and that produced on resin by the same kind of rubber he called negative, or resinous, fluid. The nature of the rubber has to be taken into account, for if glass be rubbed with cat's skin, for example, it will become charged with negative instead of positive electricity, and similarly, if resin be rubbed with cat's skin it will become positively electrified. The kind of electricity called forth depends, in fact, on some mysterious relation between the two surfaces which come into contact with one another.

Benjamin Franklin combated the hypothesis of Dufay, and suggested that there is only one electric 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.

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FIG. 2.

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Machines giving 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 в are at

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an angle of 45° to the line of the collecting combs P, 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

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