a diagrammatic way a view of the essential parts. For convenience of drawing it is shown as if the metal fieldplates A and B were affixed to the outside of an outer stationary cylinder of glass; the six carriers p, q, r, s, t, and u being attached to the inside of an inner rotating cylinder. The essential parts then are as follows:

(i.) A pair of field-plates A and B.

(ii.) A set of rotating carriers p, q, r, s, t, and u.
(iii.) A pair of neutralizing brushes n,, në made of
flexible metal wires, the function of which is
to touch the carriers while they are under the
influence of the field-plates. They are con-
nected together by a diagonal conductor, which
need not be insulated.

(iv.) A pair of appropriating brushes a, a,, which reach
over from the field-plates to appropriate the
charges that are conveyed around by the
carriers, and impart them to the field-plates.
(v.) In addition to the above, which are sufficient to
constitute a complete self-exciting machine, it
is usual to add a discharging apparatus, con-
sisting of two combs c1, c2, to collect any unap-
propriated charges from the carriers after they
have passed the appropriating brushes; these
combs being connected to the adjustable dis-
charging balls at D.

The operation of the machine is as follows. The neutralizing brushes are set so as to touch the moving carriers just before they pass out of the influence of the field-plates. Suppose the field-plate A to be charged ever so little positively, then the carrier p, touched by n, just as it passes, will acquire a slight negative charge, which it will convey forward to the appropriating brush a1, and will thus make B slightly negative. Each of the carriers as it passes to the right over the top will do the same thing. Similarly each of the carriers as it passes from

right to left at the lower side will be touched by n, while under the influence of the charge on B, and will convey a small + charge to A through the appropriating brush a. In this way A will rapidly become more and more +, and B more and more ; and the more highly charged they become, the more do the collecting combs C1 and C2 receive of unappropriated charges. Sparks will snap across between the discharging knobs at D.

The machine will not be self-exciting unless there is a good metallic contact made by the neutralizing brushes and by the appropriating brushes. If the discharging apparatus were fitted at c1, c2 with contact brushes instead of spiked combs, the machine would be liable to lose the charge of the field-plates, or even to have their charges reversed in sign whenever a large spark was taken from the knobs.

It will be noticed that there are two thicknesses of glass between the fixed field-plates and the rotating carriers. The glass serves not only to hold the metal parts, but prevents the possibility of back-discharges (by sparks or winds) from the carriers to the field-plates as they pass.

The essential features thus set forth will be found in Varley's machine of 1860, in Lord Kelvin's "replenisher" (which had only two carriers), and in many other machines including the apparatus known as Clarke's "gas-lighter." 51. Toepler's Influence Machine. In this machine, as constructed by Voss, are embodied various points due to Holtz and others. Its construction follows almost literally the diagram already explained, but instead of having two cylinders, one inside the other, it has two flat disks of varnished glass, one fixed, the other slightly smaller rotating in front of it (Fig. 37). The field-plates A and B consist of pieces of tinfoil, cemented on the back of the back disk, each protected by a coating of varnished paper. The carriers are small disks or sectors of tinfoil, to the number of six or eight, cemented to the front of the front disk. To prevent them from being worn away by rubbing against the brushes a small

metallic button is attached to the middle of each. The neutralizing brushes n,, n, are small whisps of fine springy brass wire, and are mounted on the ends of a diagonal conductor Z. The appropriating brushes a1, a2 are also of thin brass wire, and are fastened to clamps projecting from the edge of the fixed disk, so that they communicate metallically with the two field-plates. The collecting combs, which have brass spikes so short as not to touch the carriers, are mounted on insulating pillars and are connected to the adjustable discharging knobs

D1, D. These also communicate with two small Leyden jars J, J, the function of which is to accumulate the charges before any discharge takes place. These jars are separately depicted in Fig. 38. Without them, the discharges between the knobs take place in frequent thin blue sparks. With them the sparks are less numerous, but very brilliant and noisy.

To use the Toepler (Voss) machine first see that all the four brushes are so set as to make good metallic contact with the carriers as they move past, and that the

neutralizing brushes are set so as to touch the carriers while under influence. Then see that the discharging knobs are drawn widely apart. Set the machine in rotation briskly. If it is clean it should excite itself after a couple of turns, and will emit a gentle hissing sound, due to internal discharges (visible as blue glimmers in the dark), and will offer more resistance to turning. If then the knobs are pushed nearer together sparks will pass across between them. The jars (the addition of which we owe to Holtz) should be kept free from dust. Sometimes a pair of terminal screws are added at S1, S2 (Fig. 38), connected respectively with the outer coatings

of the jars. These are convenient for attaching wires to lead away discharges for experiments at a distance. If not so used they should be joined together by a short wire, as the two jars will not work properly unless their outer coatings are connected.

52. Wimshurst's Influence Machine. In this, the most widely used of influence machines, there are no fixed field-plates. In its simplest form it consists (Fig. 39) of two circular plates of varnished glass, which are geared to rotate in opposite directions. A number of sectors of metal foil are cemented to the front of the front plate and to the back of the back plate; these sectors serve both as carriers and as inductors. Across

the front is fixed an uninsulated diagonal conductor, carrying at its ends neutralizing brushes, which touch the front sectors as they pass. Across the back, but sloping the other way, is a second diagonal conductor, with brushes that touch the sectors on the hinder plate. Nothing more than this is needed for the machine to excite itself when set in rotation; but for convenience

there is added a collecting and discharging apparatus. This consists of two pairs of insulated combs, each pair having its spikes turned inwards toward the revolving disks, but not touching them; one pair being on the right, the other on the left, mounted each on an insulating pillar of ebonite. These collectors are furnished with a pair of adjustable discharging knobs overhead;