GoG The Mode of Conduction in Gases. 



Therefore 1 cathode ray moving with a velocity of 

 7 x 6*8 x 10 8 = 4'8 xlO 9 cm. per sec. makes 1/5 pairs of ions 

 in going 1 cm. in air at a irressure of 1 mm. of Hg. 



It is interesting to compare this value with that obtained 

 by Durack. He found that 1 cathode ray moving with a 

 velocity of 4 X 10 9 cm. per sec. makes '43 pairs of ions 

 per cm. In the light of more recent experiments, his 

 determination of the velocity may be corrected. In 



applying the formula v = K ^- to determine the velocity 



from the magnetic deflexion, he made— =10 7 instead of the 

 fe m 



more correct value 1*8 x 10 7 . This would make the velocity 



of his rays 1"2 x 10° cm. per sec. Assuming that the 



ionization varies inversely as the square of the velocity, for 



a velocity of 11*8 x 10 9 cm. per sec. we get a = *97. The 



value obtained in these experiments was 1\5. The agreement 



is as good as might be expected. 



I desire to thank Sir J. J. Thomson for his inspiring 



interest and advice in these experiments. 



Cams College, 

 July 20, 1911. 



LXII. The Mode of Conduction in Gases. 

 To the TJditors of the Philosophical Magazine. 

 Gentlemen, — 



IN a paper on " The Mode of Conduction in Gases illus- 

 trated by the Behaviour of Electric Vacuum Tubes " 

 that appeared in the July Number of the Philosophical 

 Magazine, Sir Oliver Lodge has put forward a theory to 

 explain how the conductivity is produced. A good many 

 theories of these phenomena have been proposed which are 

 in some points consistent and founded on experimental 

 evidence, but there are few in which so many imaginary 

 properties are attributed io the molecules. To mention 

 an instance the following passage may be quoted. " In an 

 active vacuum-tube the continual presence of such ions near 

 the cathode depends on their having been able to travel from 

 the anode ; nevertheless they are not atoms of the anode 

 itself, but are gas atoms w T hich have become positively charged 

 by contact with it, — each of them having presumably given 

 up an electron to the metal. 



" It is at the surface of the anode therefore that the sepa- 

 ration of electricities really takes place, under stress of the 

 applied E.M.F. The ions then migrate to the cathode, and 



