408 HANDBOOK OF PHYSIOLOGY. 



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 cop- 

 per 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, which is a little apparatus for 

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

 mond's (Fig. 287, D); it consists of two pieces of brass about an inch 



Fio. 284. Diagram of a Daniell's battery. 



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 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 approxi- 

 mated 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, if they are placed behind its nerve, and the connection be- 

 tween the two brass plates of the key be broken by depressing the handle 

 of the bridge, and so raising the connecting piece of metal. The key is 

 then said to be opened. 



(2.) An induced current is developed by means of an apparatus, 

 called an induction coil, and the oneemployed for physiological purposes 

 is mostly Du Bois Reymond's, the one seen in Fig. 285. 



