ELECTRICITY 507 upper comb is placed above one of the lower combs, which appears to increase the power. In both forms of the machine work is expended in turning the plates in opposition to electrical attractions and repulsions, by which mechani- cal is converted into electrical energy. It will always be found difficult to obtain good results with electrical machines in damp weather. By warming the glass insulators, however, and frequently rubbing them with a warm dry cloth, their non-conducting property may be in a degree preserved. The Holtz machine is more sensitive to moisture than the ordinary kinds. Other apparatus besides the ordinary machine are employed to develop statical elec- tricity, the electrophorus of Volta being used with advantage in some experiments. It usu- ally consists of a cake of resin about 1 in. thick, represented in fig. 9, resting upon a metallic base called the sole, and having resting upon it a metal disk with a rounded edge, and of less diameter than the resinous cake, furnished with an insulating handle. A good material for the resinous cake is composed of ten parts of shellac and one of Venice turpentine. Resin is better than glass or ebonite, because it does FIG. 9. Electrophorus. not so readily condense moisture on its surface, and this little instrument requires to be in ex- cellent condition to yield satisfactory results. In making experiments with the electrophorus the resinous cake is whipped with a catskin or rubbed with fur, which excites it negatively, and the metal disk is then laid upon it; this therefore has its natural fluid decomposed, the positive being drawn to the under and the negative repelled to the upper surface. The disk is now touched with the finger, by which it becomes positively charged from the earth, and if removed by the insulating handle will yield a spark ; an operation which may be re- peated several times before the resinous cake loses its charge. The hydro-electric machine of Sir William Armstrong, which is said to have been discovered by accident, consists of a wrought-iron plate boiler, fig. 10, standing upon four glass legs. It is about 5 ft. long by 2 ft. in diameter, provided with a gauge, O, to show the height of water. A stopcock, C, when opened allows the steam to pass through a number of tubes in the box B, containing cold water for cooling the steam The ends of the tubes are furnished with jets whose construction is such as to increase the friction, and the jets are lined with hard wood, as shown in the figure at M. A metal plate, F, armed with points, collects the elec- tricity, which is ordinarily positive, and con- veys it to the prime conductor, D. There FIG. 10. Sir William Armstrong's Hydro-electric Machine. should be a pressure of several atmospheres in the boiler, and the water should be pure, the presence of a saline solution in the escape tube being sufficient to prevent development of electricity. By changing the material of which the jets are made the kind of electricity may be varied. If a small quantity of oil of turpen- tine is introduced into the boiler, the steam will become negatively and the boiler positively electrified. The generation of electricity by this machine was at first attributed to con- densation of steam, but Faraday showed that it was caused by the friction of minute globules of water passing through the jets. Experi- ments are made with the ordinary glass-plate machine, represented in fig. 6, in the following manner: Let the rubbers be connected with the earth, the surface of the machine, especial- ly the insulating supports, having been well rubbed with a warm dry cloth. If now the glass plate is turned briskly by the winch, on ^resenting a knuckle to the prime conductor nduction takes place, the knuckle becoming charged with negative electricity. As soon as
- he tension is sufficient to overcome the resist-
ance of the air, the fluids combine with a sharp report and a spark, a stinging sensation )eing felt in the knuckle. The spark will be
