1909] SIMILAR TO ROWLAND'S METHOD. 191 



If now the needle is left positively charged, but the condenser metals 

 exchanged (commutated), so that the aluminum core is earthed and 

 the shell put in contact with the electrometer (see figure), the equa- 

 tion becomes 



V'= (Va/K){i+KVe/Va(i—e-''). 

 Let K = N/K and k' = N/K' where K' refers to the normal 

 velocity of positive ions, u. Then if ^= Vc/KVa, and k' = Vc/kV^, 

 similarly 



V=Va{i—kN)c-''\ 

 If the potential V ^=V^ at t:^ cc, 



V. = kVu/N — Ve, VJ = KVa/N + Vc, 



two equations from which both A'' and Vc may be found, if the 

 limiting potentials V^, VJ, and the electrometer current Vj, are 

 severally observed. If V^ is not obtainable, it may be computed 

 from observations at t and t^ = 2t, as 



V^=(2V—V^)/V-andVJ={2V' — V,')/V'-. 



Here however there is a difficulty as the curves begin with a double 

 inflection not yet expained. The times t^ = 2t must therefore be 

 estimated from the observations beyond the double inflections ; or 

 the rearward prolongation of the curve for those observations, to 

 meet the time axis. The initial tangents may be found in the same 

 way, but this is not necessary since their values are, respectively, 



Va{i — kN) andVa(i+kN). 



3. Data: Origin of the Electrometer Current. — The seat of the 

 chief electromotive force in the electrometer follows from the follow- 

 ing data, in which the capacities C, C, C" , Fig. i, are successively 

 removed. The currents increase in the same ratio as the reduction 

 of capacities, E being that of the electrometer. The data are (poten- 

 tials in scale parts where i cm, is equivalent to .0595 volt) , Va being 

 the fall per second : 



