Page:The American Cyclopædia (1879) Volume VI.djvu/513

 ELECTRICITY 505 lectricity to that of the hollow conductor, but the electricity which existed upon the internal irface of the latter while the conducting body was inside will now return to its external sur- face. In fact, the two bodies are related to each other in a manner similar to the conductor and cylinder in fig. 2, so that the exception to the rule is more apparent than real. An elec- trified insulated body after a time loses its elec- tricity ; this is called dissipation of charge, and takes place partly along the supports and partly through the air. Coulomb found that, other cir- cumstances remaining the same, the rate of loss is simply proportional to the charge, so that at equal intervals of time the charges form a de- creasing geometrical series, following a law sim- ilar to Newton's law of cooling. Electrical Machines. The first electrical machine was in- vented by Otto von Guericke, who was also the inventor of the air pump. A ball of sulphur turn- ed on an axis was electrified by friction with the hand, receiving negative electricity, while the positive flowed to the earth through the person. Sulphur was afterward replaced by resin, and for this the English philosopher Hawksbee sub- stituted a cylinder of glass. The hand was used as a rubber in' all these, the cushion rub- ber made of horsehair, covered with silk, being introduced by Winckler in 1 740. About the same time it is said that Boze. another German philosopher, used an insulated cylinder of tin. In 1760 Ramsden replaced the glass cylinder by a circular glass plate. Fig. 6 is a modifica- tion of Ramsden's machine in use at the pres- ent time. Between the two supports in which the glass wheel turns, and near its upper and lower edges, are two pairs of cushions, usually made of leather stuifed with horsehair and FIG. 6. Ramsden's Plate Electrical Machine. coated with bisulphide of tin or some amalgam, mixed with grease, as Kienmier's amalgam, which is composed of one part of zinc, one of tin, and two of mercury. These cushions are the rubbers for producing friction, and are con- nected with the earth by means of a metal chain or rod. Two large hollow cylinders of brass, C C, with globular ends, each supported by two glass pillars, constitute the reservoir for receiv- ing the electricity, which in this case is posi- tive. They are called the prime conductors, and are supplied with U-shaped rods of metal, furnished with points along their sides called combs, for the purpose of receiving the elec- tricity from the glass plate, the arms of the U being held upon either side. The other ends of the conductors are connected by a rod, r, from the middle of which projects another short rod terminating in a knob for delivering the spark. The action of the machine is as follows : When the plate is turned friction de- composes the fluid, positive electricity going to the glass and negative to the rubbers. The positive electricity of the plate attracted by the combs, passes to the prime conductor, while the negative upon the latter passes in a contrary direction to the plate, which in turn delivers it to the rubbers. The rubbers in con- sequence of their connection with the earth, receive positive electricity, which is powerful- ly attracted by the negative they contain, the union being again destroyed by friction and the positive fluid delivered to the prime con- ductor, where it collects until the tension be- comes greater than the confining force of the air. It is then spontaneously discharged in sparks and jets, with a crackling and hissing noise, unless taken off by a conducting body. Upon the two-fluid theory it is supposed that as much of negative fluid passes in one direc- tion as of positive in the opposite; so that a body always contains the same amount of electricity, either of one or of both kinds. Some machines, like that of Prof. Nairne, which employs a glass cylinder, are constructed with two conductors, one connected with the rubbers and the other with the points which receive the fluid from the cylinder. By con- necting the positive conductor with the earth, and insulating the rubbers, negative electricity may be obtained. A convenient method is to mount the prime conductor upon an indepen- dent stand, and connect it at pleasure either with the points or with the rubbers. A great variety of forms of machines may be con- structed, and the prime conductors may be made to receive their charge either by con- duction from the plate or rubbers, or by in- duction. Of the latter kind, one invented by Holtz of Berlin, in 1865, remarkable for its great power, is represented in fig. 7. A circular glass plate, a, is fixed in an insulating frame. In opposite sides, near the circumference, are two large orifices, which are partly covered at the back by two bands or armatures of thick paper, //', having points projecting in the opposite direction to that of the motion of a second glass plate, &, somewhat smaller than a, and made to revolve very near it. Oppo- site the face of the movable plate, which has no orifices, there are metal combs, p p', con-