DETERMINATION OF DIFFERENTIAL EXCITABILITY 281 



for one minute for recovery. In this way, records of response 

 and recovery are obtained, at intervals, say, of one minute. 



Another very effective means of producing equi-alter- 

 nating shocks is by the employment of an alternating- 

 current dynamo, driven by an electrical motor, M (fig. 172). 

 The alternating current is led to the primary of a Ruhm- 

 korffs coil in the usual manner. The motor is driven by an 

 electrical supply from the street mains, its speed being 

 adjusted by a regulation of the current, which is effected by 



m 



:; 



FIG. 172. Excitation by Equi-alternating Shocks 



M, motor, rotating armature of alternating-current dynamo, D ; R, liquid 

 rheostat, in circuit with street-mains, for regulating speed of rotation 

 of motor ; p', idle coil ; P, primary coil ; I, resonating index ; s, 

 secondary coil, in series with specimen and galvanometer. Duration 

 or excitation determined by pressure of key, K. 



n electrolytic rheostat, R. As the dynamo is provided with 

 a permanent horse-shoe magnet, the intensity of the alter- 

 nating current is determined by the speed of rotation of 

 its armature. If the speed be kept always constant, the 

 number of alternations will also be constant, and the ex- 

 iting value of the electric shocks will depend simply upon 

 he distance of the primary from the secondary. It is thus 

 possible day after day to use the same intensity of stimu- 

 lation, and thus to compare the relative excitabilities of 





