ELECTRICAL CHANGES IN MUSCLE 



255 



meniscus. If the point connected to acid be negative as compared with the 



point connected to mercury in capillary, the 



meniscus moves towards the point of the 



capillary. If the acid be positive as compared 



with the capillary, the meniscus moves away 



from the point. The extent of the excursion 



is proportional to the difference of potential. 



Since the capillary electrometer appears to have 



no latent period, and i? free from instrumental 



vibrations, it is extremely useful in recording 



the quick changes in potential occurring in the 



diphasic electrical changes that accompany 



every contraction-wave in the body. The 



excursions lend themselves well to photography, 



so that we may obtain a graphic record of 



every electrical variation, and thus determine 



its extent and its time-relations. 



It must be remembered that this instru- 

 ment is an electrometer (measurer of difference 

 of potential), and not a galvanometer (current 

 measurer). When the electrometer is connected 

 with two points at different potential, no 

 current passes through it. Hence the use of 

 non-polarisable electrodes is not so essential in 

 experiments with this instrument as when we 

 make use of the galvanometer. 



In the D'Arsonval galvanometer (Pig. 83) 

 the current is sent through a coil of fine wire 

 hung between the poles of a permanent magnet. 

 The same principle is made use of in the string Capillary electrometer. (BURGH.) 

 galvanometer of Einthoven (Fig. 84). In this 



a very delicate thread of silvered quartz <3r of platinum is stretched between the 

 poles of a strong magnet. The poles of the magnet are pierced by holes so that 



FIG. 83. 



the thread may be illumined by an electric light from one side, and from the 

 other may be observed by means of a microscope ; or a magnified image of the 



