68 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



which must here be regarded as an adequate correction for the ends and 

 the imperfect cylindricity of the condenser fog chamber. Similarly the 

 equation for the positive ionization is (negative charge) 



C\nRjR 2 d(lnV') ,d(lnV') 

 AT =- - - =K 



600 nine dt dt 



and the total ionization is therefore N+N'. 



The experiments below will show that even if the fog chamber is put to 

 earth there is a drift towards negative potential sufficiently steady as 

 to be eliminated in the mean results. 



The drift is due to a high permanent voltage which has its seat in the 

 electrometer and is not due to the lighting circuit of the building, for, 

 even when this circuit is cut out, the effect remains with undiminished 

 intensity. It will appear in Chapter VI that in the absence of radium and 

 of initial charge in the condenser the equation I a =CV a , where V a for 

 any given ionization is a constant quantity, of the same sign as the 

 voltage of the needle, applies very closely within the limits of measureable 

 V a values. Hence in the presence of radium in the core of the cylindrical 

 fog chamber and a positive charge 



/ a +6oo TT IVNev/lnRJR^C V 

 Thus in this case 



VN = *d(V-V a )/dt -V'N' = K'd(-V'- 

 and for the same V = V , to a first degree of approximation, 



numerically, as before. If the equation for N is integrated, and N /K = K, 

 since I a = CV a , V a being intrinsically negative for a negative needle, 



V = ~ Kt (V - VJK] + V a K V = e-K''(V' + VJK'} - VJK' 



where V and V are the initial positive and negative potentials. Both 

 of these are of the form 



where k is a constant, considered in the next chapter. If & = o,asin the 



V 

 present case, and V= V , the correction would be ?(*' ), which shows 



that large values of V only are admissible. 



