Wimshurst's Infiuence Machine.— This machine, due to Mr. James Winishurst, is one of the most efficient and reliable of the induc tion electric machines. It consists of two glass. discs, which in practice have varied in size from 141A inches to 7 feet in diameter. These discs suitably mounted on insulated axles are placed side by side and both are rotated, but in opposite directions. On the outer surface of each disc thin metal strips, or sectors, PA, are glued, as indicated in Fig. 3. Two adjustable metal rods, R, R, terminating in small brushes that glide over the metal strips, are supported as shown, at oblique angles to one another ; one opposite each disc. U-shaped col lectors, u u, carrying metal combs, diametric cally opposite to one another, are metallically connected to the electrodes or prime conductors, c c. These collectors are supported on metal rods that rest in what are practically Leyden jars or condensers, j J. The best position of the brushes on rods, R a, relatively to each other and to the combs is microphone, found by actual test to be virtually as shown in the figure.
The object in employing condensers, y y, is to add capacity to the prime conductors, thereby increasing the amount of electricity that can be accumulated, and hence increasing the energy of the discharge, and this use of condensers is common to all static electric machines. The action of this machine in operation is also com plicated. The Wimshurst machine is self-excit ing, that is, it starts without any externally applied electric charge, as is requisite with the Holtz machine. It has been thought that the initial charge is due to the friction of the air in the space between the two oppositely re volving plates, this space not exceeding one eighth of an inch. Apparently the metal sec tors are the equivalent of the inductors in the Holtz machine; the neutralizing rods serving to allow the repelled electricity in one sector to escape to a diametrically opposite sector on the same plate, where it in turn acts inductively on the opposite sector on the opposite plate, the free electricity when it reaches the collectors being carried. off as a discharge by the prime conductors. Frequently a small Wimshurst ma chine is used to excite a larger Holtz machine.
All static electric machines, owing to the high potentials which they develop, require ex tra precautions as regards insulation, even the damp atmosphere of a room preventing their successful operation. Some other insulating
materials, such as ebonite and gutta-percha, are less hygroscopic than glass, but these materials are not so durable as glass; hence the latter is given the preference for the plates of these ma chines. But to add to their insulating quali ties they are always covered with a shellac var nish, and are enclosed in a glass chamber or box from which moisture is extracted by sul phuric acid or other desiccators, contained in suitable vessels within the chamber. These ma chines are now usually operated by electric mo tors. Although the electro-motive force devel oped by these machines is very high, the current, owing to the high resistance of the machines, is comparatively low. Thus tests have shown the output of a Holtz machine to be, with a six inch spark, 71,000 volts and .00048 ampere, equal to 34 watts, and with a spark of 18 inches, 180, 000 volts and .0002 ampere, equal to 36 watts (Sheldon). The efficiency of the machine is, in the first instance, 27 per cent, and in the second 19.5 per cent. The current is approximately directly proportional to the rotation of the plates. The Holtz machine and others of its type are continuous current machines, and at a given speed their current output and electro motive force are constant. By efficiency is meant the energy given out by the machine as compared with that expended in driving it. (The efficiency of a dynamo electric machine is often over 90 per cent.) The electric power ex pended in driving these machines, under test, was, in the. first case, 126 watts, and in the sec ond case, 180 watts. Later tests by Sheldon on other static machines show an efficiency of 40 to 46 per cent, with an output of 10 to 12 watts.
To increase the volume of current from static electric machines, the number of plates is increased. Hence machines of the Holtz and other more or less similar types are constructed with from say 6 revolving and 6 stationary plates to 16 revolving and 16 stationary plates, or more as may be desired. In many cases the stationary plates are square or oblong sheets of glass. For therapeutic purposes, machines, giv ing at least a 12-inch spark, are considered ad visable. Plates made of micanite have been tried for static machines, but with some doubt as to their durability. See ELECTROTHERA PEU