solutions, water either in a liquid or vaporous form, the human body or animal tissues, vegetable substances, and in general, all moist or humid substances.

The worst conductors, or best non-conductors, are resins, gums, india-rubber, silk, glass, precious stones, spirits of turpentine, oils, air, and gases when perfectly dry.

Methods of Electrifying Bodies.

366. Non-conducting bodies are electrified only by friction, but conductors may be electrified either by friction, by contact, or by induction.

In order to electrify a metal it must be insulated; that is, it must be surrounded by non-conducting bodies, and it must be rubbed by an insulated body.

This may be effected by mounting the metal upon a stand of glass and rubbing it with a non-conductor, such as a piece of silk. Were the metal not insulated, the electricity would flow off to the earth as fast as generated, and were the rubbing body not a non-conductor, the electricity would flow off through the hands and arms of the experimenter.

The method of electrifying by contact depends upon the property of conductibility. If a conductor is brought in contact with an electrified body, a portion of the electricity of the latter at once flows into the former body. If the two bodies are exactly alike, the electricity will be equally distributed over both. If they differ in size or in shape, the electricity will not be equally distributed over both.

The method of electrifying bodies by induction is similar to that of magnetizing bodies by induction, and will be treated of hereafter.

Can con

The worst conductors? (366.) How are non-conductors electrified? ductors be electrified by friction? How? How are bodies electrified by contact?

Accumulation of Electricity on the Surface of Bodies.

367. Experiment shows that when a body is electrified, the electricity all goes to the surface of the body, where it exists in a thin layer, tending continually to escape. It actually does escape as soon as it finds an outlet through a conducting body.

Of the various experiments intended to show this fact, we select one that was first performed by COULOMB. He mounted a copper sphere upon an insulating rod of glass, as shown in Fig. 251. He then provided two hollow hemispheres also of copper, which, when put together, exactly

fitted the first sphere, and these he insulated by attaching them to glass handles. Having placed the hemispheres so as to cover the solid sphere, he brought the whole apparatus in contact with an electrified body till it was fully charged. On removing the apparatus from the electrified body, he separated the two hemispheres abruptly, and applied to each in turn the electrical pendulum, when he found that both were electrified. On testing the solid sphere in like manner, he could discover no trace of electricity; in other words, it was perfectly neutral.

In taking away from the body its outer coating, he had removed every particle of its electricity, which proved that the electricity was entirely upon the surface.

Another fact which indicates the same conclusion is, that a hollow and a solid sphere of the same size and of the same material, will be charged with exactly the same quantity of electricity when made to communicate with the same electrical source.

When the electric fluid is accumulated upon the surface of a body, it tends to escape with a certain force, which is named the tension.

The tension augments with the quantity of electricity accumulated. So long as it does not pass a certain limit, it is held by the resistance of the air, but if the tension passes this limit, the electricity escapes with a crackling noise and a brilliant light called the electric spark. In moist air the tension is always feeble, because the electricity is slowly conveyed away by the moisture. In a vacuum, there is no resistance to the escape of electricity, and the tension is nothing. The electricity in this case flows off as fast as generated, with a feeble light.

Influence of the Forms of Bodies.—Power of Points.

368. The distribution of electricity over the surfaces of bodies depends upon their form. If a body is spherical, the

What fact confirms COULOMB's conclusion? What is the tension? What is the electric spark? Why is the tension feeble in moist air? In a vacuum? (368.) What effect has the form of a body?

fluid is equally distributed, as may be shown by an instrument called a proof-plane.

The proof-plane consists of a disk of gilt paper attached to the end of a rod of gumlac, which insulates well. Taking the rod in the hand as shown in Fig. 252, it is applied successively at different points of the electrified surface, and after each contact it is presented to the electrical pendulum.

If the electrified body is a sphere, the same amount of attraction for the pith ball is shown, wherever the contact may be made; this shows that the proof-plane is equally charged at every point of the sphere, and consequently it is inferred that the distribution is uniform over the whole surface.

When the body is elongated and pointed, as in Fig. 252, different results are obtained. In this case the proof-plane

is more highly charged at the sharp end of the body than at any other point, showing a larger amount of electricity at the point than elsewhere. In general, it may be shown that the greater the curvature of a surface at any part, that is, the nearer it approaches a point, the greater will be the accumulation of electricity there. This shows that electricity tends to accumulate at, or to flow towards the pointed portions of bodies.

The accumulation of electricity at points gives rise to a high tension, which is sufficient to overcome the resistance of the air and to give rise to an escaping current. In fact, metallic bodies of a pointed shape soon lose the electricity imparted to them, and often the escaping current may be felt by placing the hand in front of the point. If the flow takes place in a darkened room, it may be discovered by a feathery jet of faint light.

The property of points, or the power of points, as it is called, was noticed by FRANKLIN and made use of by him in his theory of lightning-rods.

II.- PRINCIPLE OF INDUCTION.

ELECTRICAL MACHINES.

Induction.

369. If an insulated conductor in a neutral state is brought near an electrified body, the fluid of the latter acting upon that of the former, decomposes it, repelling the fluid of the same name, and attracting that of a contrary name. This operation is called INDUCTION, and it may take place not only at considerable distances, but also through non-conducting bodies, such as air, glass, and the like.

The method of electrifying bodies by induction is shown in Fig. 253. On the right of the figure is the prime conductor of an electrical machine, which, as we shall see hereafter, is charged with the positive fluid. On the left is a

What effect has a pointed form? Discuss the power of points. (369.) What is Induction?