202 



ELECTROMETRY. 



potential of two points, provided this difference does not exceed 

 o'9 volt. It consists of a tube A (Fig. 160), drawn out at the bottom 

 in a very fine point. This tube, which is open at both ends, contains 

 a column of mercury kept up by capillary action ; two platinum 

 wires a and /?, one connected with the mercury of the tube, and the 

 other with the mercury of the vessel, form the electrodes. When 

 these electrodes are connected by a conductor which contains any 



Fig. 1 60. 



convenient electromotive force, the polarisation increases the value 

 of the capillary constant of the mercury (270), and the bottom of 

 the column rises ; it is brought back to its original level, which is 

 defined by a mark on the microscope M, by exerting a pressure at the 

 top of the tube by means of a caoutchouc cylinder T. 



The electromotive force is deduced from the difference of pressure 

 measured by the manometer H. For as the total pressure is inversely 



