Page:Proceedings of the Royal Society of London Vol 60.djvu/126

Rh where q0is measured as above in grams per metre. They give an abstract of the results on their p. 742, in the shape of a table which I reproduce. Electricity at the Kew Observatory. I l l

7o = 1*6 1-9 2-5 3-7 4 -6 5-6 6-5 7-6 8*4 9-4 10-613-5 dY jdn (observed) = 502 430 400 318 252 137 184 148 112 115 118:121 ,, (calculated) = 496 442 [ 364 268 224 189 166 145 133 119 107! 85

It would appear that Elster and Geitel, like Exner, found large departures from the mean dV/dn of a group amongst its individual members.

§ 15. Elster and Geitel next proceed to investigate a possible connexion between the potential gradient and the intensity of that species of solar radiation which dissipates a negative charge on an insulated sphere of polished zinc. If I understand them rightly, they measured the mid-day intensity J of this radiation, and compared the potential gradient with several formulae in which the variable was either J or J/, where f is a “ Beleuchtungsfactor,” equal apparently to (possible hours of sunshine) /12. Taking a formula — 110 + 360a_J, where log a = O'OIOO, they give the following comparison of the results of observation and theory :—

J = 2-9 5-8 9-1 21-4 58-8 77-1 113-7 121 -9 181 -3 194 -5 268 -4 dVjdn (observed) = 447 430 368 325 198 181 138 126 120 106 102
 * ,, (calculated) = 447 425 402 330 203 171 136 132 116 114 111

The agreement seems better than in the case of Exner’s formula, and Elster and Geitel seem strongly inclined to regard ultra-violet radiation as the direct cause of variations of potential on normal quiet clear days. They consider apparently that there are only two defective links in the chain of evidence, viz.:—

(1) absolute proof that the earth is electrified negatively;

(2) proof that there is a sufficient supply at the earth’s surface of materials susceptible to the influence of ultra-violet light.

There are, of course, numerous other theories of atmospheric electricity, but none, so far as I know, admits of numerical comparison with observation.