METHODS OF ELECTRICAL STIMULATION 41 



electrometer. The platinum should never come 

 in contact with the acid. 



The electrometer should be kept short-cir- 

 cuited, except during an observation, so that 

 the capillary and the mercury in the reservoir 

 may always be connected through a conductor. 

 The short-circuit key is shown in Fig. 5, B. 

 A strip of spring brass connected with one 

 of the binding posts of the electrometer rests 

 against a second piece of brass connected with 

 the other binding post, except when depressed 

 by the finger. The point of higher potential, 

 when known, should always be connected with 

 the capillary. 



When the capillary electrometer is connected 

 with two points of unlike potential the meniscus 

 is displaced. The pressure necessary to bring 

 it back to its original position is proportional to 

 the electromotive force that displaced the me- 

 niscus. Thus by connecting the electrometer 

 with known differences of potential it may be 

 experimentally graduated. In practice, the re- 

 lation between the pressure and the potential 

 must frequently be redetermined. It is usually 

 easier to measure differences of potential, such 

 as the demarcation current of nerve or muscle, 

 by compensation (Fig. 47, p. 294). In this method 

 the electromotive force of the demarcation current 



