GALVANISM 589 FIG. 3. Cruikshank's Battery. wire, which latter, when joined to the wire connected with the copper plate in the first cup, closed the circuit. In Volta's cup bat- tery, as well as in the pile, the terminal plates were connected with plates of the opposite metal, a method adopted in accordance with the contact theory; but these additional plates have been discarded as useless. Oruikshank in 1802 modified the form of the pile by using a trough and omitting the separating pieces of cloth, which then became unnecessary. In this battery, which is represented in fig. 8, a plate of zinc and one of copper are placed together in pairs and held in vertical grooves, all the zinc plates facing in one and all the copper plates in the other direction. The connection between the pairs of plates should be imper- vious to the fluid in the trough, for the same reason that a similar condition must be ob- served in the construction of the pile. It is plainly observable that Cruikshank's battery is only a horizontal voltaic pile, possessing but little originality, and not the novelty or con- venience of Volta's crown of cups. Useful modifications of it, however, have been made. A common form, sometimes still employed, consists of a wooden trough divided into sep- arate compartments containing the exciting fluid, into each of which are suspended a zinc and a copper or a zinc and a platinum plate, from a horizontal wooden beam, the opposite elements in each compartment being connected together. The beam slides in vertical grooves in posts at the end of the trough ; by which means the plates may be raised out of or low- ered into the liquid. (See fig. 4.) They may FIG. 4. Modern Trough Battery. also be easily removed from the beam and cleaned or amalgamated with mercury, an op- eration which it is essential to perform with zinc plates which are not of pure metal ; and it not being practical to procure this, the opera- tion of umalgaination is therefore universal. It ~;s in applying metallic mercury to the I surface of the zinc plates, by which the pure zinc becomes dissolved and brought to the surface where the action of the acid is con- fined. In impure unamalgamated zinc, local polarization takes places, forming local cur- rents which greatly diminish or annul the elc- FIG. 5. Cell of Wollas- ton's Battery. tromotive force. A modification devised by Wollaston consisted in having a sheet of copper brought around one end of a zinc plate and separated from it by pieces of cork. Any num- ber of couples can be united by using a trough divided into compartments, or by employing a number of glass or earthen cups such as are represented in fig. 5. Smee's battery is formed of couples which are the reverse of Wollaston' s, there being a middle plate of platinum, or silver cov- ered with finely divided platinum (the latter form increasing the surface and giving an element of strength), with a plate of zinc on each side, not bent, however, around the end of the middle plate as in Wollaston's. It is found that this ar- rangement is better than to have the positive ele- ment in the middle. A powerful form of battery for heating purposes, in consequence of the immense quantity of elec- tricity it generates, was constructed by Prof. Hare of Philadelphia, and consists of one, or only a few simple couples,' having a great me- tallic surface. A large sheet of zinc, of seve- ral hundred square feet of surface, and a simi- lar one of copper, are separated by a piece of felt or cloth saturated with acidulated water" and then rolled together in the form of a cylin- der. (See fig. 6.) On account of its extra- ordinary heating power, it is called Hare's calorimotor or de- flagrator. All these forms of batteries, which employ two metallic elements and one fluid, when used for any con- siderable length of time, are found to be defective on ac- count of the enfee- blementof the cur- rent, which is duo to several causes, the principal be- ing : 1, decrease in chemical action in consequence of the gradual separation FrG 6 ._ Hare , 8 Ca iorimotor. of the acid by the zinc or positive element, and the accumulation of the salt which is thereby formed ; 2, the formation of local currents in the positive plate, in consequence of impurities contained in it, and interfering- with the general current ; _ 3, the production of secondary currents which flow in a contrary direction to the general
