The quality of the glass used in electrical machines is important, that containing most silica, such as the material from which ordi nary pale green bottles are made, being most suitable for this purpose.
Plate Electric Machine.—This is another form of the frictional machine. The principle of its operation is the same as that of the cylin der machine just described. It consists of a circular plate of glass, or ebonite, E, Fig. 2, in place of the cylinder. Two sets of rubbers, s, s, are placed on opposite sides of the plate, and as the plate is rotated positive electricity is deyel oped on the glass, and is collected by the prune conductor, e, virtually as in the case of the cyl inder machine.
Static Electric Induction machines depend for their action upon static in duction as exemplified in the case, for instance, of the electrophorus which may be briefly de scribed here.
Electrophorus, A simple electrophorus consists of a cake of resin or vulcanite 12 or 15 inches in diameter and one inch thick, resting on a tin or iron plate. A disc of metal, termed the cover or carrier, somewhat smaller than the calce of resin, is provided with an insulating handle. When the cake is rubbed with dry flan nel it becomes negatively electrified. If now the cover be placed on the cake, its positive electricity is attracted to the side of the cover next thc cake. The electric charge on the resin is not discharged into the cover, however, be cause, on account of the slight accidental irreg ularities of the two surfaces, the cover will be in actual physical contact with thc resin at only a few points,— perhaps at not more than three. The electricity residing in thc resin at the points of true contact will be dinharged, but not that at other points, because the resin is a non-con ductor, and so its electricity cannot pass directly to the covering plate by conduction. Now if the cover be touched by the finger, its negative elec tricity will escape to earth. If now the cover be lifted from the cake it will contain free posi tive electricity which may be used for charging a condenser or Leyden jar. The cover may
again be placed on the cake and the foregoing process may be repeated a number of times be fore the charge in the resin will be entirely ex hausted. The cake when electrified, with the disc in its place, and undisturbed, will retain its charge for weeks.
A number of induction machines based on the foregoing principle have been devised, among the best known being the Holtz and the Wimshurst, which are sometimes termed con tinuous electrophori.
Holtz's Electric Induction Machine.—This consists of two glass plates or discs about two feet in diameter, placed side by side and sep arated by a very small air space. One plate is fixed; the other is rotated, the fixed plate being slightly larger. The moving plate is mounted on a well-insulated axle. On diametrically opposite points of the fixed plate there are two sector-shaped holes or windows. On the back of the fixed plate, at the windows, are glued paper inductors or armatures, blunt tongues from which they are caused to pass through the windows until they nearly touch the moving plate, which must be rotated in the direction opposite to that in which the tongues point. Opposite the inductors there are placed rnetal combs attached to brass rods or electrodes, which carry brass balls that at their terminals are movable to and from one another. In start ing the machine the two balls are brought to gether and a negative charge from a rubbed ebonite rod is given to one of the inductors; then when the plate is rotated and the balls are separated, sparks jump across the space. The action of the machine is very complicated and need not be gone into here further than to say that in effect it is virtually similar to that of the electrophorus, the initial charge imparted to the armature inducing opposite electricity in the rotating disc, which in turn delivers a charge to the metal comb, which charge by successive ad ditions as the disc rotates is augmented until it attains a potential of 50,000 volts and more. These discs are rotated at a speed varying from 120 to 450 revolutions per minute. See ELEC