508 HANDBOOK OF l'H YSIOLOGY. 



Galvanic currents are usually obtained by the employment of a continuous 

 current battery such as that of Daniell, by which an electrical current which 

 varies but little in intensity is obtained. The battery (fig. 320) consists of a 

 positive plate of well-amalgamated zinc immersed in a porous cell, containing 

 dilute sulphuric acid ; and this cell is again contained within a large 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 sulphate into copper 



Fig. 320. Diagram of a Daniell's battery. 



and sulphuric acid. The former is deposited upon the 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, \vhich is a little appa- 

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

 Reymond's (fig. 321) ; it consists of two pieces of brass about an inch long, in 

 each of which are two holes for wires and binding screw, to hold 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 battery are brought to the inner binding screws, 

 and the bridge connects them, the current passes across it and back to the 

 battery. Wires are connected with the outer binding screws, and the other 

 ends are joined together for about two inches, but, being covered except at 

 their points, are insulated ; the uncovered points are about an eighth of an 

 inch apart. These wires are the electrodes, and the electrical stimulus is applied 

 to the muscle through them, if they are placed behind its nerve. When the 

 connection between the two brass plates of the key is broken by depressing the 

 handle of the bridge, the key is then said to be opened. 



An induced current is developed by means of an apparatus, called an indue- 



