1903.] The Hon. B. J. Stnitt on Electrical Conduction. 293 



WEEKLY EVENING MEETINd, 

 Friday, April 24, 1 903. 



His Gracr Thb Duke of Northumberland, K.G. D.C.L. F.R.S., 

 President, in the Chair. 



The Hon. R. J. Strutt, M.A., B^eHow of Trinity College, 

 Cambridge. 



Some Recent Investigations on Electrical Conduction. 



We have here a gold-leaf electroscope, which you can see projected 

 on the screen. I charge it with electricity, and the leaves remain 

 divergent. If, however, I touch the knob of the electroscope with 

 my finger, or with any other conductor, the leaves at once collapse. 



Now the knob is at all times in contact with the air of the room. 

 We may infer therefore that if the air of the room conducts at all 

 under the condition of this experiment, it can be only to a very 

 slight extent. 



If, however, air be submitted to very much greater electrical 

 stress, quite a different state of things sets in. I have here a tube, 

 containing rarefied air, which we will expose to a powerful electric 

 stress, by connecting its terminals to those of an induction coil. 

 You see at once that its insulation breaks down, and that it conveys 

 the electric current, which produces brilliant and complex luminous 

 efiects in it. 



These phenomena are of great interest and importance, and some 

 light has been thrown on their cause and nature by recent investiga- 

 tions. But I do not propose to enter into such difficult questions to- 

 night. We shall confine our attention to the behavour of gases under 

 small electromotive forces, such as are insufficient to produce lumin- 

 ous discharge. 



I have explained that under these conditions the conductivity is 

 very slight, or the leaves of the electroscope could not remain 

 divergent. If, however, we expose the air to Rontgen rays, an im- 

 mediate and most striking change in its electrical behaviour takes 



You will see, in the gallery of the theatre, a bulb capable of 

 emitting Rontgen rays. I charge the electroscope, and as soon as 

 the bulb, many yards away, is set in action, the leaves collapse, 

 showing that they have lost their charge. The air of the room, 

 traversed by Rontgen rays, has lost its power of insulation, and the 

 charge of the electroscope quickly leaks away through it. Almost 

 immediately after the rays are turned off, the air recovers its insulat- 

 ing power, and as you see, the electroscope is again able to retain its 



