164 Mr. G. W. Todd : Further Experiments on the 



in the paper quoted *, showing the relation between the product 

 of pressure and mobility and the pressure for the positive 

 ions, suggested that at pressures a little lower than those for 

 which mobilities had been determined, something less than 

 the molecule of the gas might be carrying the charge. 

 Owing to the fact that the dimensions of the measuring appa- 

 ratus became of the order of about fifty free paths of the gas 

 molecules at these lowest pressures, it was impossible to 

 draw definite conclusions as to the mass of the carrier from 

 measurements of the mobility, so it was decided to construct 

 an apparatus in which it was hoped to make the ions oscillate 

 over a distance of 20 cm. instead of over 5 cm. 



Fig-. 1. 



Apparatus. — An idea of the apparatus may be gathered 

 from fig. 1. e was a carefully insulated metal plate sur- 

 rounded by an " earthed " guard-ring, and connected to a 



* Phil. Mag. Nov. 1911. 



