OBSERVATION BY RHEOTOME ON ELECTRIC RESPONSE 49 



Here, the prong is kept down by a spring S, the circuit being 

 re- made, as soon as the breadth of the striker B has passed 

 over the projecting rod that is to say, in 'Oi second (fig. 38). 

 The interval of time be- 

 tween the actions on the 

 two keys, by which the 

 two different electrical cir- 

 cuits are broken in suc- 

 cession, can be gradually 

 increased, by increasing 

 the angle between A and B. 

 The key K,, as already 

 said, controls the electro- 

 magnet, which, on its 

 release, instantaneously 

 effects . the mechanical 



FIG. 38. Enlarged View of Balanced 

 Keys 



K,, actuated by rod A, K a by B. 



stimulation of the tissue. 

 The rotation of the rheo- 

 tome-disc does not at once 



become uniform, on the starting of the motor, but attains 

 this when .one revolution has been completed. Therefore 

 the experimental observations are not made till the speed 

 has become steady. By pressing the key K 3 during the first 

 revolution (fig. 37), the break-action of A on K t is postponed. 

 K 3 is then opened, and during the next revolution, stimulation 

 is effected. 



In order to obtain the galvanometric effect of excita- 

 tion at definite short intervals of time after the stimulus 

 has been applied, the galvanometer short circuit, as stated 

 before, is removed at those definite intervals. The adjust- 

 ment of the striking-rod B, in relation to A, enables us to 

 open the short circuit, for 'Oi of a second, at increasing 

 intervals. When the rod B is at a distance of I cm. from A, 

 the short circuit is removed after 'Oi second, when at 2 cm. 

 after '02 second, and so on. Thus, in the arrangement just 

 described, the galvanometer is short-circuited, except at those 

 definite intervals required for observation. In a second 



E 



