GALVANISM. 



405 



be packed together in a glass tube, so that their similar metallic faces shall 

 all look the same way, and be pressed tightly together at each end by metallic 

 plates, it will be found that one extremity of the pile is positive and the 

 other negative. Such a series will last more than twenty years, but it re- 

 quires as many as 10,000 pairs to afl'ord sparks visible in daylight, and to 

 charge the Leyden jar. 



Fig. 335 represents a pair of these piles, so arranged as to produce what 

 has been called a perpetual motion. Two piles, P N, are placed in such a 

 position that their poles are reversed, and between them a light pendulum, 

 vibrating on an axis and insulated on a glass pillar. This pendulum is alter- 

 nately attracted to one and then to the other, and thus rings two little bells 

 connected with the positive and negative poles. 



The galvanic batteries in practical use at the present time diflfer consider- 



ably in form and efficiency, but the principle of construction in all is the same 



as that of the original voltaic pile. 



^ .^ ,^ A very effective FlO. 336. 



Descnbe the •' 



trough battery, arrangement known 



as the trough bat- 

 tery, is represented in Fig. 336. 

 This consists of a trough of wood 

 divided into water-tight cells, or 

 partitions, each cell being arranged 

 to receive a pair of zinc and copper 

 plates. The plates are attached to 

 a bar of wood, and connected with 

 one another by metaUic wires, in 

 such a way that every copper plate 

 is connected with the zinc plate of 

 the next cell The battery is excited by means of dilute sulphuric acid poured 

 into the cells, and the current of electricity is directed by wires soldered to the 

 extreme plates. "When the battery is not in use the plates may be raised from 

 the trough by means of the wooden bar. 



The battery by which Sir Humphrey Davy effected his splendid chemical 

 discoveries was of this form, and consisted of two thousand double plates of 

 copper and zinc, each plate ha\'ing a surface of thirty-two square inches. 

 Now, however, by improved arrangements, we can produce with ten or 

 twenty pairs of plates, effects every way superior. 



Fig. 337. 



