-398] Electrical Induction. 409 the air, and allow electricity to escape. It is in fact observed, that metallic bodies provided with a point quickly lose their electricity, and, if the hand be held over such a point, a sort of wind or draught is felt. If this takes place in darkness, a kind of luminous brush appears on the top of the point. This property of points, placed on electrified conductors, of allowing electricity to escape, has been called the power of points; and in electrical experiments we meet with numerous instances where it comes into play. CHAPTER II. ACTION OF ELECTRIFIED BODIES ON BODIES IN THE NATURAL STATE; INDUCED ELECTRICITY. ELECTRICAL MACHINES. 398. Electricity by influence or induction.-An insulated conductor, charged with either kind of electricity, acts on bodies in a natural state placed near it, in a manner analogous to that of the action of a magnet on soft iron, that is, it decomposes the neutral fluid, attracting the opposite, and repelling the like kind of electricity. The action, which is a consequence of the attractions and repulsions of the two electricities, and which is exerted not only through air but also through insulating bodies like air, glass, resins, etc., is said to take place by influence or induction. The phenomena of induction may be demonstrated by means of the experiment represented in fig. 314. On the right hand of the figure is the conductor of the electrical machine, which, as we shall afterwards see, is charged with positive electricity; on the left is a + Fig. 314. brass cylinder, insulated by being placed on a glass support, and provided with small pith ball pendulums, suspended by linen threads, which are conductors. When the cylinder of the machine is brought near this conductor, the pendulums are found to diverge but to unequal extents, the greatest divergence being met with at the ends. Near the middle the pith balls do not diverge at all; the electricity is, therefore, accumulated at the ends, and the middle is in the neutral state. If, moreover, a sealing-wax rod which has been rubbed with flannel be approached to the pendulums nearest the electrical machine, they will be repelled, showing that they are charged with the same electricity as the rubbed sealing wax, that is, negative electricity. If, in like manner, a glass rod, which has been rubbed with silk, be approached to the other end of the -398] Electrical Induction. 411 cylinder, the pendulums are also repelled, which shows that they are charged with positive electricity. The electricities thus separated are equal in quantity, for if the machine is removed all the pendulums cease to diverge, since the two electricities have recombined, and the body is restored to the neutral state. This electrifying by influence, or induction as it is called, which is produced by an electrified body or bodies in the neutral state, explains a host of phenomena. In order to explain all its effects, it is important to inquire what takes place when, in the above experiment, the insulated cylinder is placed for a short time in contact with the ground, while it is still under the influence of the machine. Suppose, for instance, the further end be placed in contact with the ground, the positive electricity will escape, while the negative remains held by the attraction of the opposite electricity of the machine. If now connection with the ground be interrupted and the cylinder be moved away from the influence of the machine, the pendulums will diverge, and, as can be easily verified, owing to their being charged with negative electricity. Even if the end nearest the machine be connected with the ground the result is still the same. The negative electricity does not pass into the ground; it is the positive which still escapes; the negative being attracted by the contrary electricity of the machine, on interrupting the communication with the earth, the cylinder remains charged with negative electricity. Thus a body can be charged with electricity by induction as well as by conduction. But, in the latter case, the charging body loses part of its electricity, which remains unchanged in the former case. The electricity imparted by conduction is of the same kind as that of the electrified body, while that excited by induction is of the opposite kind. To impart electricity by conduction, the body must be quite insulated, while, in the case of induction, it must be in connection with the earth, at all events, momentaneously. What has here been said has referred to the inductive action exerted on good conductors. Bad conductors are not so easily acted upon by induction, owing to the great resistance they present to the circulation of electricity, but, when once charged, the electric state is more permanent. This is analogous to what is met with in magnetism; a magnet instantaneously evokes magnetism in a piece of soft iron; but this is only temporary, and depends on the continued action of the magnet; a magnet magnetises steel with far greater difficulty, but this magnetism is permanent. ELECTRICAL MACHINES. 399. Ramsden's electrical machine. The first electrical machine was invented by Otto von Guericke, the inventor also of the air-pump. It consisted of a sphere of sulphur, which was turned on an axis by means of the hand, while the other, pressing against it, served as a rubber. Fig. 315. Resin was afterwards substituted for the sulphur, which, in turn, Hawksbee replaced by a glass cylinder. In all these cases the hand served as rubber; and Winckler, in 1740, first introduced cushions of horsehair covered -399] Electrical Machine. 413 with silk as rubbers. At the same time, Bose collected electricity disengaged by friction, on an insulated cylinder of tin plate. Lastly, Ramsden, in 1760, replaced the glass cylinder by a circular glass plate, which was rubbed by cushions. The form which the machine has now is but a modification of Ramsden's original machine. Between two wooden supports (fig. 315) a circular glass plate, P, about a yard in diameter, is suspended by an axis passing through the centre, and which is turned by means of a glass handle. The plate revolves between two sets of cushions or rubbers, I, I, of leather or of silk, one set above the axis and one below, which, by means of screws, can be pressed as tightly against the glass as may be desired, by which means the plate becomes electrified on both sides. The plate also passes between two brass rods shaped like a horse-shoe, and provided with a series of points in the sides opposite the glass, which are called the combs; these rods are fixed to larger metallic cylinders, C, which are called the conductors. The latter are insulated by being supported on glass feet, and are connected with each other by a smaller rod. The action of the machine is founded on the excitation of electricity by friction, and on the action of induction. By friction with the rubbers, the glass becomes positively, and the rubbers negatively electrified. If now the rubbers were insulated, they would receive a certain charge of negative electricity which it would be impossible to exceed, for the tendency of the opposed electricities to reunite would be equal to the power of the friction to decompose the neutral fluid. But the rubbers communicate with the ground by means of bands of tinfoil, EE, fixed to the supports, and, consequently, as fast as the negative electricity is generated, it passes off. The positive electricity of the glass acts then by induction on the conductor, attracting the negative fluid. The conductors thus lose their negative electricity, and remain charged with positive fluid. The plate accordingly gives up nothing to the conductors; in fact, it only abstracts from them their negative fluid. As thus described, the electrical machine yields only positive electricity; it may, however, be arranged so as to give negative electricity. For this purpose the four feet of the table are insulated by being placed on thick plates of resin, of glass, or of sulphur, and the conductors are connected with the ground by a metallic chain. This allows the electricity of the positive conductors to escape, |