478 Dr. C. Chree : Results obtained at Kew Observatory with 



the axis of abscissse (" saturated " current condition). A 

 further stage then presents itself which does not concern us. 

 Mache and v. Schweidler * assert, what seems to be now 

 generally allowed, that the conditions in the Elster and Geitel 

 instrument as ordinarily used are those of the "saturated'' 

 current. If so, then the true formula is 



dYldt+A=0, (3) 



where A is proportional to the number of ions being gener- 

 ated in unit of time in unit of space surrounding the 

 dissipator. This last formula leads to 



A=(V -V ( )A, (4) 



where V is the original value of the potential, and V* its 

 value after time t. 



Let us compare this with the corresponding results from 

 the equation assumed by Elster and Geitel, viz. 



«=(l/01og(V /V,) (5) 



Writing this a= -(1/0 log {1 + (V - V )/V }, 



we see that as a first approximation, provided (Vt— V )/V 

 be small — as is normally the case at Kew — we have 



« = (V0(V -V,)/V (6). 



The formulae (2) and (5) would apply to the ideal Elster 

 and Geitel instrument in which the capacity of the electro- 

 scope is negligible, or n = 0. The result calculated from the 

 right-hand side of (5) would represent a/100 in Elster and 

 GeiteFs notation. We see, however, that so long as the loss 

 of charge in time t is small, we may to a close degree of 

 approximation replace (5) by (6). If, then, the true law of 

 loss of charge be not (2) but (3), the calculated value of 

 a/ 100 is very approximately the true value of A/V , 

 and so varies approximately as A — the number of ions being 

 generated in unit of time — provided V is constant. 



In the Kew observations V was not really a constant, but 

 it seldom departed much from its mean value, which was 

 about 180 volts for both the positive and negative charges. 



It is unquestionably desirable that no doubt should exist as 

 to the exact physical significance of observational quantities. 

 It is doubtful, however, whether any one of the quantities 

 measured in atmospheric electricity observations is wholly 



* Die Atmospharische Ulektrizitat, pp. 63, 64. 



