PRACTICAL PHYSIOLOGY 55 



Capillary electrometer. The capillary elec- 

 trometer consists essentially of mercury, which is 

 forced by pressure from behind for a certain dis- 

 tance into a glass tube drawn out to a capillary 

 termination ; the free end of the capillary is filled 

 with dilute sulphuric acid and dips into a vessel 

 containing the same fluid. The capillary is ob- 

 served with a microscope. If the mercury and the 

 sulphuric acid be now connected with wires which 

 are charged with electricity there is produced a 

 movement of the mercury in the direction which the 

 current would take i.e., from positive to negative 

 the extent of movement of the meniscus being, 

 roughly speaking, proportioned to the difference 

 of potential. From the direction and extent of 

 the movement the direction and electromotive 

 force of the current can therefore be gauged. 



Join a pair of non-polarisable electrodes up in 

 circuit with a capillary electrometer and Daniell 

 cell through a rheochord and commutator in the 

 manner shown in the diagram (Fig. 38), but with 

 a piece of blotting paper moistened with salt 

 solution placed across the electrodes instead of 

 the muscle shown in the figure. Put a short- 

 circuiting key between the electrometer and the 

 electrodes. Have the short-circuiting key shut 

 at first so that the electrometer is short-circuited, 

 and the battery key open. Bring the mercury 

 meniscus into the field of the microscope. Now 

 open the short-circuiting key. If the electrodes 

 are themselves without current there will be no 

 effect on the electrometer ; but usually there is a 



