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Electrophorus.

467

lined on its under surface with tinfoil, and provided with an insulating glass handle. This is called the cover. The mode of working this apparatus is as follows :-All the parts of the apparatus having been well warmed, the cake, which is placed in the form, or rests on a metal surface, is briskly flapped with a catskin, as shown in fig. 382, by which it becomes charged with negative electricity. The cover held by the insulating handle is then placed on the cake. The negative electricity of the cake, acting thus inductively on the cover, attracts positive electricity to the lower surface, and repels negative to the upper. If now this upper surface be touched by the finger, as shown in fig. 383, the negative electricity passes out into the ground, and the disc only retains positive electricity. Now when the cover is raised by one hand by means of the insulating handle, and the other hand is brought near it, a smart spark passes,

Fig. 382.

due to the recombination of the positive of the disc with the negative produced by its induction in the hand (fig. 384).

Replacing the disc upon the cake, this again exerts its inductive action, for it is such a bad conductor that the electricity does not pass off to the cover, and, if the same operations be repeated, a succession of such sparks may be obtained even after the lapse of some time. The retention of electricity is greatly promoted by keeping the cake in the form, and placing the cover upon it, by which the access of air is hindered. Instead of a cake of resin, a disc of gutta percha, or vulcanised cloth, or vulcanite, may be substituted; and of course, if any material which becomes positively

electrified by friction be used as the cake, the cover acquires a negative charge.

426. Gold leaf electroscope. The gold leaf electroscope, also called Bennett's electroscope, from the name of its inventor, is a small but delicate apparatus for ascertaining whether a body is electrified, and if so with what kind of electricity it is charged. It consists of a tubulated glass shade (fig. 385), the neck of which is closed by a cork. In this is fitted a brass rod terminating at the top in a knob, and at the bottom in two strips of gold leaf. The neck, the cork, and the upper part of the shade are coated with a thick layer of

sealing-wax varnish, which is nothing more than a solution of sealing-wax in spirits of wine. The object of this coating is to improve the insulating qualities of the glass. Glass is, indeed, a bad conductor, but it is very hygroscopic: that is, it readily attracts aqueous vapour from the air, and thus becomes coated with a layer of moisture, which renders its surface a conductor (418). When covered with varnish this evil is removed, for varnishes, which are usually made of resin, are not hygroscopic.

The air in the inside is dried by quicklime, or by chloride of

- 426]

Gold Leaf Electroscope.

469 calcium, or by pumice stone soaked in strong sulphuric acid, and on the inside of the shade there are two strips of gold leaf communicating with the ground.

When the knob is touched with a body charged with either kind of electricity, the leaves diverge; usually, however, the apparatus is charged by induction in the following manner :—

If an electrified body-a stick of sealing-wax rubbed with flannel, for instance - be brought near the knob, it will decompose the natural electricity of the system, attracting the electricity of the opposite kind to the knob and retaining it there, and repelling the electricity of the same kind to the gold leaves, which consequently diverge. In this way, the presence of an electrical charge is ascertained, but not its quality.

Το ascertain the kind of electricity the following method is pursued :-If, while the instrument is under the

influence of the body,

Fig. 385.

which we will suppose has a negative charge, the knob be touched by the finger, the negative electricity decomposed in induction passes off into the ground, and the previously divergent leaves will collapse: there only remains positive electricity retained in the knob by induction from the sealing-wax. If now the finger be first removed, and then the electrified body, the positive electricity previously retained by the sealing-wax will spread over the system, and cause the leaves to diverge with positive electricity. If now, while the system is charged with positive electricity, a positively electrified body, as, for example, an excited glass rod, be approached, the leaves will diverge more widely; for the electricity of the same kind will be repelled to the extremities. If on the

contrary, an excited shellac rod be presented, the leaves will tend to collapse, the electricity with which they are charged being attracted by the opposite kind. Hence we may ascertain the kind of electricity, either by imparting to the electroscope electricity from the body under examination, and then bringing near it a rod charged with positive or negative electricity; or the electroscope may be charged with a known kind of electricity, and the electrified body in question brought near the electroscope.

It has been proposed to use the electroscope as an electrometer, or measurer of electricity, by measuring the angle of divergence of the leaves. This is done by placing behind them a graduated scale. There are, however, many objections to such a use, and it isarely employed for this purpose.

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Electrical Spark.

471

CHAPTER III.

ELECTRICAL EXPERIMENTS.

427. Electrical spark.-One of the first experiments which is made by those who see an electrical machine at work for the first time is that of taking from it an electrical spark by bringing the

hand near the conductor. The positive electricity of the conductor acting inductively on the neutral electricity of the body, decomposes it, repelling the positive and attracting the negative. When the tension of the opposed electricities is sufficiently great to overcome the resistance of the air, they recombine with a smart crack