We may now try to understand the working of influence machines, which are really mechanically worked electrophori. There are various types of such machines, but the one in most general use in this country is that known as the Wimshurst machine, Fig. 4, and we will therefore confine ourselves to this. It consists of two circular plates of varnished glass or of ebonite, placed close together and so geared that they rotate in opposite directions. On the outer surfaces of the plates are cemented sectors of metal foil, at equal distances apart. Each plate has the same number of sectors, so that at any given moment the sectors on one plate are exactly opposite those on the other. Across the outer surface of each plate is fixed a rod of metal carrying at its ends light tinsel brushes, which are adjusted to touch the sectors as they pass when the plates are rotated. These rods are placed at an angle to each other of from sixty to ninety degrees, and the brushes are called neutralizing brushes. The machine is now complete for generating purposes, but in order to collect the electricity two pairs of insulated metal combs are provided, one pair at each end of the horizontal diameter, with the teeth pointing inward towards the plates, but not touching them. The collecting combs are fitted with adjustable discharging rods terminating in round knobs. The principle upon which the machine works will be best understood by reference to Fig. 5. In this diagram the inner circle represents the front plate, with neutralizing brushes A and B, and the outer one represents the back plate, with brushes C and D. The sectors are shown heavily shaded. E and F are the pairs of combs, and the plates rotate in the direction of the arrows. Let us suppose one of the sectors at the top of the back plate to have a slight positive charge. As the plates rotate this sector will come opposite to a front plate sector touched by brush A, and it will induce a slight negative charge on the latter sector, at the same time repelling a positive charge along the rod to the sector touched by brush B. The two sectors carrying the induced charges now move on until opposite back plate sectors touched by brushes C and D, and these back sectors will receive by induction positive and negative charges respectively. This process continues as the plates rotate, and finally all the sectors moving towards comb E will be positively charged, while those approaching comb F will be negatively charged. The combs collect these charges, and the discharging rods K and L become highly electrified, K positively and L negatively, and if they are near enough together sparks will pass between them. [Illustration: FIG. 5.--Diagram to illustrate working of a Wimshurst Machine.]