chap, xii.] USE OF FROG-INTERRUPTER. 131 



primary coil of an induction machine I (Fig. 67). 

 Now, the instant the key is closed the current passes 

 through the coil, producing, therefore, an induced 

 current in the secondary, through the interrupter, and 

 through the galvanometer, deflecting the needle. But, 



o O O 



as we see, the same current that deflects the needle 

 produces a single induced current, which may be led 

 to the muscle, and stimulates it to contraction. As 

 soon as it contracts, contact is broken at p and q, the 

 current is interrupted,- and the needle returns to zero. 

 Now, practically the production of the induced 

 current is simultaneous with the closing of the 

 primary current, so that the muscle is stimulated at 

 the same instant that the primary circuit is closed. 

 If, therefore, the muscle contracted precisely at the 

 moment of stimulation, it would by its contraction 

 break the primary circuit the same moment it was 

 closed, and no current would circulate in the 

 primary circuit ; therefore no deflection of the 

 needle of the galvanometer would occur. But as a 



^j 



matter of ' fact the muscle does not contract the 

 moment it is stimulated, consequently for a very 

 short time the primary current affects the galvano- 

 meter, and then the current is interrupted. The 

 needle, therefore, is deflected to a certain extent and 

 then returns to zero. The short time is the time that 

 elapses between the moment of the muscle receiving 

 its stimulus and the moment of its contraction, i.e. 

 the period of latent stimulation. Thus, the con- 

 ditions mentioned at the beginning of this chapter 

 are fulfilled ; a constant stream acts on the galvano- 

 meter for a very short time, so that the amount of 

 deflection is the measure of the time during which the 

 current acted, that is, the measure of the period of 

 latent stimulation. 



One further point about the apparatus is to be 

 noted. The muscle gets only a single shock ; it 



