an Elster and Geitel Electrical Dissipation Apparatus. 47 T 



§ 2. The values of o + and a_, the percentage losses per 

 minute of positive and negative charges, have been calculated 

 from the formula 



i i r V/ i vi 



where Y denotes the initial, V* the final potential in the 

 dissipation experiment, V ' the initial and V/ the final potential 

 in the leakage experiment, both experiments being supposed 

 to last t minutes, n denotes the ratio (capacity of electro- 

 scope alone)/(joint capacity of dissipator and electroscope). 

 Unfortunately, n is a quantity which it is not easy to 

 determine accurately, and while high precision in the value 

 of n is unimportant in the leakage term inside the long 

 bracket, supposing the insulation to be good, it is of course 

 important in the factor (1— n)~ l outside the bracket. When 

 dealing with comparative results from the same instrument, 

 Elster and Geitel, in some at least of their earlier work,, 

 omitted the factor 1 — n, using the notation E instead of a. 

 The fact that E and a represented something more than a 

 mere difference in notation was overlooked in the preparation 

 of the tables of dissipation results published in the Kew 

 Reports for 1907, 1908, and 1909. The values assigned thero 

 to a + and a_ really answer to E + and E_ in Elster and 

 Geitel's notation, and thus presumably require multiplication 

 by (1— n)' 1 to be comparable with values published for 

 other stations. The value obtained for n in direct experi- 

 ments at Kew was 0*3, the corresponding value of (1 — n)~ x 

 being 1/0*7 or 1'43. 



So long as the insulation is kept satisfactory the ascription 

 of a wrong value to n practically alters all values of a + and 

 a_ in the same proportion, and so is without influence on 

 any conclusions that depend only on relative values. Such 

 an error is for instance without effect on values of a_/a + or 

 on the annual variation observed in a_ or a + . 



§ 3. There is another question affecting the interpretation 

 of the results. 



Elster and Geitel assume " dissipation " to follow the law 



dV/dt±*Y = (2) 



This is at least approximately true in air so long as the 

 potential gradient is small, a being a measure of the 

 conductivity. As the gradient, however, is raised, the curve 

 in which abscissae represent gradient and ordinates current 

 departs markedly from a straight line, and then throughout 

 a considerable gradient range remains practically parallel to- 



