26 HAND-BOOK OF PHYSIOLOGY. 



The kind of current employed may be, for the sake of clearness, treated 

 of under two heads: (1) The continuous current, and (2) The induced 

 current. (1) The continuous current is supplied by a battery such, as 

 that of Daniell, by which an electrical current which varies but little in 

 intensity is obtained. The battery (Fig. 277) consists of a positive plate of 

 well-amalgamated zinc immersed in a porous cell, containing dilute sul- 

 phuric acid; and this cell is again contained within a larger copper vessel 

 (forming the negative plate), containing besides a saturated solution of 

 copper sulphate. The electrical current is made continuous by the use of 

 the two fluids in the following manner. The action of the dilute sulphuric 

 acid upon the zinc plate partly dissolves it and liberates hydrogen, and 

 this gas passes through the porous vessel and decomposes the copper sul- 

 phate into copper and sulphuric acid. The former is deposited upon the 



FIQ. 277. Diagram of a DanielTs Battery. (After Balfour Stewart.) 



copper plate and the latter passes through the porous vessel to renew the 

 sulphuric acid which is being used up. The copper sulphate solution is 

 renewed by spare crystals of the salt which are kept on a little shelf 

 attached to the copper plate and slightly below the level of the solution 

 in the vessel. The current of electricity supplied by this battery will 

 continue without variation for a considerable time. Other continuous- 

 current batteries such as Grove's may be used in place of Daniell's. The 

 way in which the apparatus is arranged is to attach wires to the copper 

 and zinc plates and to bring them to a key, which is a little apparatus 

 for connecting the wires of a battery. One often employed is Du Bois 

 Keymond's (Fig. 280, D); it consists of two pieces of brass about an inch 

 long, in each of which are two holes for wires and binding screws to fix 

 them tightly; these pieces of brass are fixed upon a vulcanite plate, to 

 the under surface of which is a screw clamp by which it can be secured 

 to the table. The interval between the pieces of brass can be bridged 

 over by means of a third thinner piece of similar metal fixed by a screw 

 to one of the brass pieces and capable of movement by a handle at right 

 angles, so as to touch the other piece of brass. If the wires from the 



