1903.] on Some Becent Investigations on Electrical Conduction. 295 



the action of Rontgen rays, a gas containing an element of higb 

 molecular weight, such as iodine, gives a current out of all proportion 

 greater. 



I wish now to return to a question which was too lightly passed 

 over at the beginning of this lecture. 



I said that the leakage of electricity through the air in its normal 

 condition was very slight. But 



the existence of such a leakage P 



has been incontestably proved. ^ 



The difficulty of establishing -^ 



the conclusion is this. It is neces- 

 sary to make use of some solid 

 insulating support for the gold 

 leaves of the electroscope. Now, 

 if the leaves are observed to 

 slowly collapse, it is difficult, with 

 ordinary arrangements, to deter- 

 mine whether this leakage is really 

 through the air or whether it takes 

 place through the insulating sup- 

 port. This ambiguity of the ex- 

 periment has been ingeniously 

 overcome by Mr. C. T. R. Wilson, 

 of Cambridge. His method was to 

 carry the further end of the insu- 

 lating support on a piece of metal 

 which was at a higher potential 

 than the gold leaves. Any failure 

 in the insulation would then 

 cause the leaves to diverge more 

 than at [first. It was found, 

 however, that in fact the leaves 

 collapsed in the course of a few 

 hours. There could, therefore, 

 be no doubt whatever that the 

 charge was escaping through the 

 air. 



I have here an experiment 

 which shows the same thing, 

 though perhaps not quite so satis- 

 factorily. There are two pairs of gold leaves, a, h (Fig. 1), in ali 

 respects similar. These are supported by clean ebonite insulators, 

 c, d, exactly alike for each. The left-hand pair, however, hangs 

 immediately from the ebonite support, while the right-hand one 

 hangs from the support by a long wire e. The right-hand charged 

 system, therefore, has much better access to the air than the left-hand 

 one. And in the course of half-an-hour you will see that its leaves 

 have collapsed much further, in spite of its greater electrical capacity. 

 Vol. XVII. (No. 97.) x 



Fig. 1. 



