CAPILLARY ELECTROMETER. 239 



The position of the thread of mercury in the capillary 

 is controlled pneumatically by means of a small mercury 

 pressure apparatus. The meniscus of the mercury is observed 

 through a microscope with a power of 100 diameters. 



The terminals of the electrometer are provided with a 

 short-circuiting key, which is to be kept closed when the 

 instrument is not in actual use. 



The less the diameter and length of the capillary, so much 

 the greater will be the sensitiveness of the instrument. 



Only small electrical pressures may be exerted upon the 

 mercury in the capillary, and an ordinary cell must on no 

 account be placed in its circuit unless high resistances are 

 included at the same time, as bubbles of H gas and crystals 

 of mercury sulphate will separate with the current from a 

 single Leclanche, thus rendering the capillary useless. 



The movements of the meniscus are sudden and dead beat, 

 and the mercury always moves away from the plus pole in 

 proportion to the electrical pressure exerted. 



The instrument is used as a pressure detector and not for 

 measuring current, and the quickness of its response makes it 

 the only means for detecting small and rapid variations in the 

 electrical condition of animal tissues. 



The condition and changes of potential already referred to in 

 the previous exercise are readily confirmed by its aid. 



