288 M. G. Lippmann on the Connexion between 



These currents may be produced and measured in a simple 

 manner. 



The apparatus described above (page 285) was used for the 

 measurement. The poles a and /3 were respectively connected 

 with a reflecting galvanometer of 2000 turns. To make an 

 experiment, the reservoir of mercury A was either raised or low- 

 ered; the level of the mercury G G' was altered thereby, and a 

 throw of the needle produced. Alterations in level of a few mil- 

 limetres were sufficient to make measurements; otherwise cur- 

 rents are obtained so strong that a needle which is not astatic 

 strikes against the stops. When the level is raised, the surface 

 of contact of the mercury and the acid in M increases. The 

 galvanometer then indicates a current which proceeds through 

 the dilute sulphuric acid from the increasing electrode to the 

 other. On sinking, the direction of the current is reversed. 



Experiments were made with various heights of lift and with 

 different glass tubes. Let a be the observed deflection of the 

 galvanometer, / the height of the lift, r the radius of the tube ; 

 all the numbers found satisfied the condition 



= const. =« ; 



Zirrl 



that is, the quantity of electricity disengaged is proportional to the 

 increase of surface^ and independent of the form. 



This quantity of electricity was now determined in absolute 

 electromagnetic measure. The above constant a denotes the 

 deflection in scale- divisions for an increase of surface of 1 square 

 millim. It was found that a =0*55 on the scale. To express 

 this number in electromagnetic measure, the ends of the galva- 

 nometer wire were connected with the ends of a coil above which 

 a second coil was placed ; this latter was traversed by a current 

 whose intensity amounted to 4*5 in absolute electromagnetic 

 measure. The potential of the two coils on each other (as 

 given by Professor Kirchhoff) amounted to 23,223,000. When 

 the current in the second coil was opened, an induced current 

 was produced in the first which deflected the mirror through 

 57 divisions. Moreover the resistance of the galvanometer 

 together with the induced coil was compared with a known re- 

 sistance by means of Wheatstone's bridge, and was found to be 

 equal to 78 x 10 10 divisions. Hence the quantity of electricity 

 for a one- division deflection amounted to 



23223000 x 4-5 

 78xl0 10 x57 ; 



and thus for a deflection of 0*55, or an increase in surface of 1 

 square millim., the quantity of electricity disengaged is in abso- 



