LIPPMANN'S CAPILLARY ELECTROMETER. 



203 



as the diameter of the tube at the bottom and proportional to the 

 capillary constant, the difference of pressure relative to the variation 

 of the capillary constant produced by a given electromotive force is 

 proportional to the original pressure. 



The following table was calculated for a column of mercury 

 of 75 centimetres. The electromotive force is expressed in fractions 

 of that of a Daniells' element. 



The increase of pressure is at first proportional to the electro- 

 motive force ; it then passes through a maximum for an electromotive 

 force of 0*9 ; the capillary constant attains then 1*47 times its normal 

 value. 



Electromotive 

 Force. 



0*0 1 6 



0*040 

 0*I09 

 0*I40 



0*197 

 0*269 

 0*364 

 0*450 



Increase 

 Pressure. 



Cm. 



2-15 



4*0 

 8*9 

 n*i 

 13*1 

 14*8 

 18*85 



23-5 

 27*05 



It is important that the electrode a which corresponds to the 

 capillary tube should be always negative, in order to avoid oxidation 

 of the mercury. It is also necessary to wait until the capillary tube 

 is well moistened by acid, which is effected by causing the column 

 of mercury to oscillate slightly. 



The capacity of the electrometer is proportional to the surface 

 of the mercury in the drawn-out tube. Although this surface is 

 very small, its capacity is far greater than that of the instruments 

 described above ; for a polarized surface acts always as a condenser 

 with plates very near each other. The capacity of polarization of 

 the platinum is at least 0*1 microformed for a square millimetre, 

 and therefore equal to that of a sphere of about 900 metres radius. 

 The capacity of mercury must be a quantity of the same order. 



