280 



COMPARATIVE ELECTRO-PHYSIOLOGY 



contact with the brushes, the upper will then be connected 

 with the even strip and the lower with the odd. Thus the 

 direction of the current will be reversed, and rapid rotation 

 of the disc will give rise to equi-alternating currents in the 

 primary of the induction coil. This will in turn induce 

 equi-alternating induction currents in the secondary, the 

 intensity of which can, as already said, be varied within wide 

 limits by appropriate changes of distance between the 

 primary and the secondary. The number of strips in the 

 apparatus used is fifty, and when the disc is rotated, by 



FIG. 171. Method of Direct Effect of Excitation by Equi- 

 alternating Shocks 



R, rotating reverser, in circuit with primary coil, P. Duration of stimula- 

 tion determined by metronome, M. s, secondary coil in series with 

 specimen and galvanometer. 



means of an electrical motor, at a rate of one revolution 

 per second, there will be fifty alternations of current in a 

 second. The duration of the application of the stimulating 

 shock to the tissue is regulated by a metronome, which 

 completes the primary circuit for a definite short length of 

 time. When the metronome, M, is so adjusted as to complete 

 the circuit for *5 second, then a stimulus of that duration will 

 be imparted at each stroke. A second interrupting key, not 

 shown in the figure, is included in the circuit When this 

 key is closed, a single beat of the metronome gives a stimu- 

 lating shock of *5 second's duration. The key is now opened 



