126 



PHYSIOLOGICAL PHYSICS. 



[Chap. XI. 



sator. 



being properly arranged in circuit, as shown (Fig. 65), 

 the disc of the instrument is turned so that the 

 platinum wheel rests on the zero point of the wire. 

 The current from the muscle is then allowed to go 

 through the galvanometer, and a 

 deflection results. The disc is then 

 slowly turned, so that the zero 

 point is carried away from the wheel. 

 This permits the Daniell current 

 access to the galvanometer, and the 

 needle slowly returns as the disc 

 moves. "When the needle is again 

 at zero of the scale, the disc is 

 allowed to remain where it is, and 

 then the new position of the platinum 

 wheel r is read off. The distance 

 now between zero of the wire and 

 r is proportional to the strength of 

 the branch current from the Daniell, sent through the 

 galvanometer to compensate for the muscle current. 



Effect of electrotomis on electromotive 

 force. With the aid of the galvanometer, then, it 

 has been found that muscles and nerves give rise to 

 an electric current, that is, develop electromotive 

 force. It has been mentioned towards the close of 

 chapter viii., that the passage of a constant current 

 of electricity through a nerve alters its electromotive 

 force, but, because it involves the use of the galvano- 

 meter, it was left to this chapter to show how this is 

 proved. The arrangement is precisely that already 

 described ; a long nerve is, however, required. Let the 

 zinc troughs be placed in connection with the galvano- 

 meter, a key being interposed, and let the nerve be 

 placed with the transverse section of one end on one 

 clay pad, and let a part of the longitudinal surface 

 near that end touch the clay of the other trough. 

 This leaves the other end of the nerve free to be laid 



