a discharge is felt by persons standing near, or may even affect electroscopes, or yield sparks. This action. known as the "return-shock," is due to influence. For in the presence of a charged conductor a charge of opposite sign will be induced in neighbouring bodies. and on the discharge of the conductor these neighbouring bodies may also suddenly discharge their induced charge into the earth, or into other conducting bodies. A "return-shock" is sometimes felt by persons standing on the ground at the moment when a flash of lightning has struck an object some d'stance away.

LESSON IV. Conduction and Distribution of Electricity

30. Conduction. Toward the close of Lesson I. we explained how certain bodies, such as the metals, conduct electricity, while others are non-conductors or insulators. This discovery is due to Stephen Gray; who, in 1729, found that a cork, inserted into the end of a rubbed glass tube, and even a rod of wood stuck into the cork, possessed the power of attracting light bodies. He found, similarly, that metallic wire and pack-thread conducted electricity, while silk did not.

We may repeat these experiments by taking (as in Fig. 25) a glass rod, fitted with a cork and a piece of wood. If a bullet or a brass knob be hung to the end of this by a linen thread or a wire, it is found that when the glass tube is rubbed the bullet acquires the property of attracting light bodies. If a dry silk thread is used, however, no electricity will flow down to the bullet.

Gray even succeeded in transmitting a charge of electricity through a hempen thread over 700 feet long, suspended on silken loops. A little later Du Fay succeeded in sending electricity to no less a distance than 1256 feet through a moistened thread, thus proving the conducting power of moisture. From that time the

classification of bodies into conductors and insulators has been observed.

This distinction cannot, however, be entirely maintained, as a large class of substances occupy an intermediate ground as partial conductors. For example, dry wood is a bad conductor and also a bad insulator; it is a good enough conductor to conduct away the highpotential electricity obtained by friction, but it is a bad conductor for the relatively low-potential electricity of small voltaic batteries. Substances that are very bad

Fig. 25.

conductors are said to offer a great resistance to the flow of electricity through them. There is indeed no substance so good a conductor as to be devoid of resistance. There is no substance of so high a resistance as not to conduct a little. Even silver, which conducts best of all known substances, resists the flow of electricity to a small extent; and, on the other hand, such a non-conducting substance as glass, though its resistance is many million times greater than any metal, does allow a very small quantity of electricity to pass through it. In the

following list, the substances named are placed in order, each conducting better than those lower down on the list.

A simple way of observing experimentally whether a body is a conductor or not, is to take a charged goldleaf electroscope, and, holding the substance to be examined in the hand, touch the knob of the electroscope with it. If the substance is a conductor the electricity will flow away through it and through the body to the earth, and the electroscope will be discharged. Through good conductors the rapidity of the flow is so great that the discharge is practically instantaneous. Further information on this question is given in Lesson XXXIII.

31. Distribution of Charge on Bodies. If electrification is produced at one part of a non-conducting body, it remains at that point and does not flow over the surface, or at most flows over it excessively slowly.

Thus if a glass tube is rubbed at one end, only that one end is electrified. Hot glass is, however, a conductor. If a warm cake of resin be rubbed at one part with a piece of cloth, only the portion rubbed will attract light bodies, as may be proved by dusting upon it through a piece of muslin fine powders such as red lead, lycopodium, or verdigris, which adhere where the surface is electrified. The case is, however, wholly different when a charge of electricity is imparted to any part of a conducting body placed on an insulating support, for it instantly distributes itself all over the surface, though in general not uniformly over all points of the surface.

32. The Charge resides on the Surface. A charge of electricity resides only on the surface of conducting bodies. This is proved by the fact that it is found to be immaterial to the distribution what the interior of a conductor is made of; it may be solid metal, or hollow, or even consist of wood covered with tinfoil or gilt, but, if the shape be the same, the charge will distribute itself precisely in the same manner over the surface. There are also several ways of proving by direct experiment this very important fact. Let a hollow metal ball, having an aperture at the top, be taken (as in Fig. 26), and set upon an insulating stem, and charged by sending into it a few sparks from an electrophorus. The absence of any charge in the interior may be shown as follows: In order to observe the nature of the electrification of a charged body, it is convenient to have some means of removing a small quantity of the charge as a sample for examination. To obtain such a sample, a little instrument known as a proof-plane is employed. It consists of a little disk of sheet copper or of gilt paper fixed at the end of a small glass rod. If this disk is laid on the surface of an electrified body at any point, part of the charge flows into it, and it may be then removed, and the sample thus obtained may be examined with a gold-leaf electroscope in the ordinary way. For some

purposes a metallic bead, fastened to the end of a glass rod, is more convenient than a flat disk. If such a proofplane be applied to the outside of our electrified hollow ball, and then touched on the knob of an elect roscope, the gold leaves will diverge, showing the presence of a

charge. But if the proof-plane be carefully inserted through the opening, and touched against the inside of the globe and then withdrawn, it will be found that the inside is destitute of electrification. An electrified pewter mug will show a similar result, and so will even a cylinder of gauze wire.