gence increases with the potential. Similarly the diminution of the divergence indicates the loss of electricity.

In order to increase the capacity of conductors and obtain stronger sparks, the prime conductor used formerly to be connected with large hollow metallic cylinders suspended from the ceiling by silk, which were called secondary conductors. It is simpler and less cumbersome to connect the conductor with the inner coating of a Leyden jar, the outer coating of which is connected with the room. If the potential of the machine should exceed that which the jar can support, a cascade arrangement may be used (§ 59).

89. The unit jar (§ 84) furnishes a very simple means of measuring the yield of the machine, and of investigating the conditions which modify it. We have already said that this yield is independent of the pressure of the rubbers when once there is sufficient contact between them and the glass. The yield is likewise independent of the capacity of the conductors, provided the causes of loss are the same. It is proportional to the surface of the glass which passes between the rubbers; it is, therefore, proportional to the length of the rubbers; thus, if one pair of rubbers is used instead of two, the yield is reduced to one half. The yield is further proportional within wide limits to the speed of rotation of the plate.

In some machines only a single pair of rubbers and a single comb is used, with the object of pushing back the limit at which discharge is produced between the conductor and the rubber. A greater difference of potentials is obtained, but a lower yield.

90. Induction Machines-Replenisher.-We shall commence the description of these machines by that of Lord Kelvin's replenisher, which Figs. 82 and 83 represents in about its ordinary size.

A and B (Figs. 82 and 83) are two portions of a cylinder with the same axis, O, both insulated, and each of them acting alternately both as inductor and collector. In the interior are two curved metal plates, P and Q, which are fixed by a cross-piece of ebonite to an ebonite rod, T, so that by means of a milled head, E, they can be made to rotate within the cylinders A and B. Four springs, a, b, c, d, placed at the same distance from the axis, are touched in succession by the projecting parts of the plates P and Q; a and b communicate with A and B respectively, while c and d are connected with each other. The action of the replenisher is

essentially the same as that of the two metal jars and balls discussed

91. Holtz's Machine. - The action of Holtz's machine is analogous, except that the increase of the charge is continuous instead of intermittent.

It consists of two glass plates (Fig. 84) placed parallel at a small distance from each other; one is fixed by means of wooden knobs supported by glass rods, the other is movable about a horizontal axis. Besides the central aperture, through which passes the axis of the movable plate, the fixed plate has two apertures or windows at the ends of the same diameter. Along one edge of each of these windows is an armature of paper provided with a tongue, also of paper, terminating in a point in the centre of the opening. Opposite the armatures, on the other side of the free movable plate are two combs connected with two small insulated conductors, P and N. By an insulated handle it is possible to vary at will the distance of the two knobs, P and N, which will be called the poles of the machine. Two sliding knobs, P' and N', may be brought into contact with the corresponding poles, so that either one or the other may be put to earth.

To get the machine to work, it must first be primed. The two poles, P and N, are brought into contact, and the movable plate being put in rotation by the handle, so as to move towards the points, an ebonite plate or stick of sealing-wax charged by friction with negative electricity is brought near one of the armatures. When the machine is in action a peculiar hissing sound is heard; from the comb, near which the ebonite was held, a luminous sheet

spreads in a direction opposite to the motion of the plate, this is the positive comb (§ 20); every point of the negative comb is tipped by a small star of light. The electrified ebonite may then be withdrawn, and the rod which connects the two poles drawn away; a brush discharge then passes between the two poles as long as the rotation continues. It is readily shown that the electricity of each of the poles is of the opposite kind to that which

escapes from the corresponding comb. The action stops if the knobs P and N are drawn too far apart.

The brush discharge depends upon the small capacity of the conductors. In order to increase this capacity, the two conductors, P and N, may be connected with two Leyden jars, K and H, the outer coatings of which are connected with each other. The two jars form thus a cascade between the two poles, and each of them has half the difference of potentials of the two poles (§ 59). If the outer coatings are connected to earth, the two conductors are at

equal and opposite potentials. When jars are used, the sparks are not continuous, as with the brush, but they follow each other at regular intervals, and are much more luminous, denser, and produce a louder crack. With a machine the plate of which is 60 cm. in diameter they may attain a length of 20 cm.

Most of these machines, as now constructed, are double; they have two fixed plates close to each other. The two movable plates, which are mounted on the same axis, rotate on the outside. The armatures are placed opposite each other, those which are similarly electrified being together; the same U-shaped comb surrounds the two movable plates. The action of the machine is in no way altered, except that the yield is double.

It is sometimes difficult to start the action of the machine, especially in damp weather; it then will not begin unless the ebonite plate is strongly electrified. The glass plate is usually made to rotate with a velocity of eight to ten turns a second.

In order to explain the action of the machine and to make the figures clearer, we will suppose the glass plates replaced by concentric cylinders (Fig. 85).

The circumference represents the movable cylinder; A' and B' the two paper armatures; the fixed cylinder is not shown, as it C has scarcely other function than that of supporting the armatures; Aa and Bb are the two conductors represented as in actual

contact.

a b

FIG. 85.

B'

Suppose the plate to be at rest. The ebonite plate C charged with negative electricity is brought near the armature A'. It acts inductively on the conductor AB; positive electricity is attracted towards A, and negative towards B; but owing to the points the positive electricity escapes on to the glass at A and negative at B until equilibrium is established.

When the plate is turned, the positively electrified glass is carried away from A, and the negatively electrified glass from B, so that the conductor AB is again left subject to the inductive action of the ebonite, which, acting as before, causes a stream of positive electricity to escape from the comb A, and a stream of

negative from B. Each part of the plate, as it passes A, thus receives a positive charge, and, as it passes B, a negative charge, until, after half a turn, the whole plate is electrified as indicated by

Fig. 86. Henceforward the armatures come into play: the paper tongues that project from them through the windows in the fixed plate graze the surface of the approaching parts of the + revolving plate, and thus the armature A' acquires a negative charge, and B' a positive charge. The electric force due to these charges acts in the same way as that due to the electrified ebonite, so that the inductive

-B'

When the

action on the conductor AB is increased, and a greater discharge of electricity-positive at A, negative at B-takes place on to the surface of the revolving plate. The consequence is that the armatures, in their turn, become more strongly charged, or acquire a greater difference of potentials, and that the action already described goes on still more actively, so that, after a few turns of the plate, the electrified ebonite may be removed and the machine continues to act. As described, the upper half of the plate carries positive electricity from left to right, and the lower half carries negative electricity from right to left, which is electrically the same thing. This transfer, due to the motion of the plate, is compensated by a flow of positive electricity from B to A through the conductor, and of negative electricity from A to B. difference of potentials between the armatures has attained a sufficiently high value, the electric flow through the conductor continues, even if the knobs P and N are separated, and it then becomes evident as a stream of sparks. If the knobs are separated too far, the action of the conductor ceases, and there being now-no electrical interchange between the combs and the revolving glassplate as it passes them, negative electricity is carried round to the positive armature B', and positive electricity to the negative armature A'. The electrification of the armatures is thus lessened, and may be destroyed altogether, or even inverted. When the knobs are quickly put into contact again, if the electrification has only been diminished, or if it has been inverted in sign, the action