METHODS OF ELECTRICAL STIMULATION 45 



ence of potential between post and post 1 will 

 be practically one-twentieth the electromotive 

 force of the element. Thus when the sliding 

 contact is at post 1, the capillary electrometer 

 receives one-twentieth the electromotive force of 

 the element. By moving the slider from post 1 

 towards post 0, any desired fraction of this one- 

 twentieth may be measured by the electrometer. 



The Simple Key. A copper bar with hard rubber 

 handle is pivoted at one end in a brass post with 

 binding screw for electrical 

 connection (Fig. 8). Near 

 the other end of the bar is 

 a platinum pin, which, 

 when the key is closed, 

 rests upon a platinum plate 



borne Upon a Second bind- Fig. 8. The simple key ; about 



three -eighths the actual size. The 

 ing post. wire spring which presses the bar 



The contact bar is held the contact plate is n 



against the contact plate 



partly by its own weight and partly by a wire spring 

 not shown in Fig. 8. When it is desired to break the 

 circuit the contact bar is turned back. 



Many experiments in physiology require stimuli of 

 uniform intensity. Variations in the make or break of 

 the current due to faults in the contacts of the key in 

 the primary circuit are a frequent source of error. 

 With the key described here the break in the circuit 

 may be made practically uniform. 



