ELECTRICAL CHANGES IN MUSCLE 227 



ordinary galvanometer. For this purpose we may employ either the capillary , I, , , 

 meter or the string galvanometer of Einthoven. 



The capillary electrometer is an instrument for recording and measuring diP- 

 of potential, lhat is to say, if connected with two 

 points, it measures the force which would make a 

 current flow between these two points if they were 

 connected by a wire. Its structure is very simple. It 

 consists of a glass tube drawn out to a fine capillary 

 point. This tube with the capillary is filled with 

 mercury. The point dips into a wide tube containing 

 dilute sulphuric acid, at the bottom of which is a 

 little mercury. Two platinum wires fused into the 

 glass and dipping into the mercury serve as terminals. 

 When the instrument is used, the meniscus of the 

 mercury in the capillary at its junction with the acid 

 is observed under the microscope, or a magnified 

 image of it is thrown on a screen with the aid of the 

 electric light. If now the capillary and acid be con- 

 nected with two points, it will be observed that any 

 difference in the potential of these two points causes 

 a movement of the 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 is free from instru- 

 mental vibrations, it is extremely useful in recording 

 the quick changes in potential occurring in the 



diphasic electrical changes that accompany every CapiUary e lwteometer. (Bru....) 

 contraction- wave in the body. The excursions lend 



themselves well to photography, so that we may obtain a graphic record of c\ TV 

 electrical variation, and thus determine its extent and its time-relations. 



It must be remembered that this instrument is an electrometer (measurer of differ- 

 ence of potential), and not a galvanometer (current measurer). W T hen the electrometer 



FIG. 81. 



Fro. 83. 



is connected with two points at different potential, current passes into it for a frarti. 

 of a second, and polarises the surface of the mercury, so that it takes up a new pos 



