G H

G H, which is separately insulated. The pins E, F rise out of the back of the fixed plates A and C, at unequal distances from the axis. The piece K is parallel to G H, and both of them are furnished at theirends with small piecesof flexible wire that they may touch the pins E, F in certain points of their revolution. From the brass piece M there stands out a pin I, to touch against a small flexible wire or spring which projects •C sideways from the rotating plate B when it comes opposite A. The wires are so adjusted by bending that B, at the moment when it is opposite A, communicates with the ball D, and A communicates with C through GH; and half a revolution later C, when B comes opposite to it, communicates with the ball D through the contact of K with F. In all other positions A, B, C and D are completely disconnected from each other.

Nicholson thus described the operation of his machine: — " When the plates A and B are opposite each other, the two fixec plates A and C may be considered as one mass, and the revolving plate B, together with the ball D, will constitute another mass All the experiments yet made concur to prove that these two masses will not possess the same electric state. . . .The redundant electricities in the f«a««*« under consideration will be unequally distri buted ; the plate A will have about ninety-nine parts, and the plate C one; and, for the same reason, the revolving plate B will have ninety-nine parts of the opposite electricity, ana the ball D one The rotation, by destroying the contacts,, preserves this unequa (JHtribution, and carries B from A to C at the same time that the tai K connects the ball with the plate C. In this situation, the clec tricity in B acts upon that in C, and produces the contrary state by virtue of the communication between C and the ball; whicl last must therefore acquire an electricity of the same kind with tha of the revolving plate. But the rotation again destroys the contac and restores B to its first situation opposite A.

Here, if we attend to the effect of the whole revolution, we shall find that the electrii states of the respective masses have been greatly increased ; for th< ninety-nine parts in A and B remain, and the one part of electricity in C has been increased so as nearly to compensate ninety-nine part: of the opposite electricity in the revolving plate B, while the com munication produced an opposite mutation in the electricity of the ball. A second rotation will, of course, produce a proportion.! augmentation of these increased quantities; and a continuance o FIG. 3. — Nicholson's Revolving Doubter. urning will soon bring the intensities to their maximum, which is iimt«lbyanexplo8ionbctwcenthcplates"(^M-''''a'w-> 1788, p. 405). Nicholson described also another apparatus, the " spinning ondenscr," which worked on the same principle. Bennet and Nicholson were followed by T. Cavallo, John Read, B.JJ/., Johnenbcrger, C. B. De'sormcs and J. N. P. Hachcttc doubler. nd others in the invention of various forms of rotating loublcr. A simple and typical form of doubler, devised in 1831 >y G. Belli (fig. 4), consisted of two curved metal plates between which revolved a pair of jails carried on an insulatng stem.

Following the nomenclature usual in connexion with dynamos we may speak of the conducors which carry the initial charges as the field plates, and of the moving conducors on which are induced he charges which are subsequently added to those on the field plates, as the carriers. The wire which Fl(J 4._Belti's Doubler. connects two armature plates for a moment is the neutralizing conductor. The :wo curved metal plates constitute the field plates and must bave original charges imparted to them of opposite sign. The rotating balls are the carriers, and arc connected together for a moment by a wire when in a position to be acted upon inductively by the field plates, thus acquiring charges of opposite sign. The moment after they are separated again. The rotation continuing the ball thus negatively charged is made to give up this charge to that negatively electrified field plate, and the ball positively charged its charge to the positively electrified field plate, by touching little contact springs. In this manner the field plates accumulate charges of opposite sign. Modern types of influence machine may be said to date from 1860 when C. F.

Varley patented a type of influence machine which has been the parent of numerous subsequent Varley,B forms (Brit. Pat. Spec. No. 206 of 1860). In it the machiae. field plates were sheets of tin-foil attached to a glass plate (fig. 5). In front of them a disk of ebonite or glass, having carriers of metal fixed to its edge, was rotated by a winch. In the course of their rotation two diametrically opposite carriers touched against the ends of a neutralizing conductor so as to form for a moment one conductor, and the moment afterwards these two carriers were insulated, one carrying away a positive charge and the other a negative. Continuing their rotation, the positively charged carrier gave up its positive charge by touching a little knob attached to the positive field plate, and similarly for the negative charge carrier. In this way the charges on the field plates were continually replenished and reinforced. Varley also constructed a multiple form of influence machine having six rotating disks, each having a number of carriers and rotating between field plates. With this apparatus he obtained « fiflVTW sparks 6 in. long, the initial source ] of electrification being a single Daniell cell. Varley was followed by A. J. I.

Toepler, who in 1865 constructed J F _Varl -s Machinc. an influence machine consisting of two disks fixed on the same shaft and rotating in the same direction. Each disk carried two strips of tin-foil extending nearly over a semi-circle, and there were two field T kr plates, one behind each disk; one of the plates was mKhine. positively and the other negatively electrified. The carriers which were touched under the influence of the positive field plate passed on and gave up a portion of their negative charge to increase that of the negative field plate; in the same