great that the discharge is practically instantaneous. Further information on this question is given in Lesson XXIII.

28. Distribution of Electricity on Bodies.—If electricity 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. If a warm cake of resin be rubbed at one part with a piece of cloth, only the portion rubbed will attract light bodies. 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.

29. 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. 18), 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 electricity 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 disc of sheet copper or of gilt paper fixed at the end of a small glass rod. If this disc is laid

on the surface of an electrified body at any point, part of the electricity 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 disc. If such

a proof-plane be applied to the outside of our electrified hollow ball, and then touched on the knob of an electroscope, 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 electricity. An electrified pewter mug will show a similar result, and so will even a cylinder of gauze wire.

30. Biot's experiment.-Biot proved the same fact

in another way. A copper ball was electrified and insulated. Two hollow hemispheres of copper, of a larger size, and furnished with glass handles, were then placed together outside it (Fig. 19). So long as they did not come into contact the charge remained on the

inner sphere; but if the outer shell touched the inner sphere for but an instant, the whole of the electricity passed to the exterior; and when the hemispheres were separated and removed the inner globe was found to be completely discharged.

31. Further explanation.-Doubtless the explanation of this behaviour of electricity is to be found in the property previously noticed as possessed by either kind of electricity, namely, that of repelling itself; hence it retreats as far as can be from the centre and remains

D

upon the surface.

An important proposition concerning the absence of electric force within a closed conductor is proved in Lesson XX. ; meanwhile it must be noted that the proofs, so far, are directed to demonstrate the absence of a free charge of electricity in the interior of hollow conductors. Many other experiments have been devised in proof. Thus, Terquem showed that a pair of gold leaves hung inside a wire cage could not be made to diverge when the cage was electrified. Faraday constructed a conical bag of linen

gauze, supported as in Fig. 20, upon an insulating stand, and to which silk strings were attached, by which it could be turned inside out. It was charged, and the charge was shown by the proof-plane and electroscope to be on the outside of the bag. On turning it inside out the electricity was once more found outside. Faraday's most striking experiment was made with a hollow cube, measuring 12 feet each way, built of wood, covered with tinfoil, insulated, and charged with a powerful machine, so that large sparks and brushes

were darting off from every part of its outer surface. Into this cube Faraday took his most delicate electroscopes; but once within he failed to detect the least influence upon them.

32. Applications.—Advantage is taken of this in the construction of delicate electrometers and other instruments, which can be effectually screened from the influence of electrified bodies by enclosing them in a thin metal cover, closed all round, except where apertures must be made for purposes of observation. It has also been proposed by the late Prof. Clerk Maxwell to protect buildings from lightning by covering them on the exterior with a network of wires.

33. Apparent Exceptions.-There are two apparent exceptions to the law that electricity resides only on the outside of conductors. (1) If there are electrified insulated bodies actually placed inside the hollow conductor, the presence of these electrified bodies acts inductively and attracts the opposite kind of electricity to the inner side of the hollow conductor. (2) When electricity flows in a current, it flows through the substance of the conductor. The law is limited therefore to electricity at rest, that is, to statical charges.

34. Faraday's "Ice-pail" Experiment.-One experiment of Faraday deserves notice, as showing the part played by induction in these phenomena. He gradually lowered a charged metallic ball into a hollow conductor connected by a wire to a gold-leaf electroscope (Fig. 21), and watched the effect. A pewter icepail being convenient for his purpose, this experiment is continually referred to by this name, though any other hollow conductor-a tin canister or a silver mug, placed on a glass support-would of course answer equally well. The following effects are observed :- Suppose the ball to have a + charge: as it is lowered into the hollow conductor the gold leaves begin to diverge, for the presence of the charge acts inductively, and attracts