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MUSCLE-NERVE PHYSIOLOGY 



mersed in a porous cell containing dilute sulphuric acid; and this cell is again contained 

 within a large copper vessel (forming the negative plate) containing 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 and sulphuric acid. The former is deposited 

 upon the copper plate, and the latter passes through the porous vessel to renew the sulphuric 



FIG. 315. Du Bois Raymond's Key. 



acid which is being used up. The copper-sulphate solution is renewed by 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 cell will 

 continue without variation for a considerable time. Other cells, such as the dry cell 

 (which, however, is adapted to open-circuit work) may be used in place of Daniell's. The 



FIG. 316. Mercury Key. 



way in which the apparatus is arranged is to attach wires to the copper and zinc plates, and 

 to bring them to a key, connecting the wires of the battery. One often employed is Du 

 Bois Reymond's, figure 315. It consists of two pieces of brass about an inch long, in each 

 of which are two holes for wires and binding-screws, to hold them tightly. These pieces 

 of brass are fixed upon a vulcanite plate to the under surface of which is attached a screw 



