THE CAPILLARY ELECTROMETER 



155 



now sent through the capillary it is found that the mercury moves in the 

 direction of the current. This movement is due to the altered surface 

 tension brought about by the current. Thus supposing the current passes 

 along the capillary from mercury to acid, the surface tension falls and the 

 mercury moves down the tube. If the current travel from acid to mercury 

 the surface tension rises, and the mercury moves up the tube until a new 

 position is found in which the external pressure is again balanced by the 

 capillarity. By increasing the pressure on the surface of the mercury the 

 mercury may be again driven down the tube till the original position is once 

 more reached. The difference in level of the mercury in the pressure apparatus 

 is a measure of the capillarity, and therefore of the difference of potential. By 

 measuring the pressure when the position of the mercury in the capillary is 

 brought back to its initial position while a current is still passing through it, 

 the instrument may be graduated and can then be directly used as a measurer 

 of small currents. Its great advantage lies in that its electrical capacity is 

 very small, and it can thus show very rapid changes of potential. There is 

 no latency with the instrument and no after-oscillation. 



It is to be noted that this instrument measures electrical pressure, i.e. 

 potential, whereas the galvanometer is a current-measurer. 



Experiment 6. — Employ the capillary electrometer for the follow- 

 ing experiment. Pith a frog, excise its heart and place it on a pair 

 of unpolarisable electrodes with the apex on one electrode and the 

 base on the other. 



Fig. 122. — Frog's Heaet. Diphasic Variation. 



Simultaneous photogram of a single beat (black line) and of the accompanying electrical change, 

 indicated by the level of the black area, which shows the varying level of mercury in a Capillary 

 electrometer. The base of the ventricle is connected with the mercury, i. First, phase, base 

 negative to apex. II, Second phase, apex negative to base. (Waller.) 



To prevent the contact between electrodes and heart shifting during 

 the beats the connection between each electrode and heart should be 

 made by a thread moistened in normal saline. The electrodes are con- 

 nected to the electrometer through a Du Bois key, so that the electro- 

 meter can be short-circuited at any time. 



If the mercury be watched it will be found to move with each beat 

 of the heart. 



