94 ELECTRONS AND IONS [CH. ix. 



hypothetical, because it was an assumption that each 

 droplet contained only one ion. Nevertheless, that 

 appears to be the truth, since the charge found on 

 each was 3 x 10~ 10 electrostatic units. 



A further method applied by Professor J. J. Thom- 

 son, and described in the Philosophical Magazine for 

 March, 1903, was used in a determination of the 

 maximum current passing between electrodes in 

 ionised air. 



It may possibly be of assistance to some if we 

 quote here an explanatory remark drafted by Mr. 

 G. Owen : 



Thomson's Deductions. 



" What Professor Thomson succeeded in doing was 

 to measure the charge on the negative ion produced 

 in air by the influence of Rontgen rays. The ques- 

 tion might naturally be asked : What grounds had 

 Professor Thomson for drawing a general conclusion 

 concerning the mass of an electron from a deter- 

 mination of the charge on an ion produced in air by 

 one particular agency? 



" The answer is as follows : The negative ions 

 produced in a gas were known to have the same 

 properties irrespective of the source of their pro- 

 duction f For instance, the negative ions produced 

 by X-rays, radium rays, and ultra-violet light, travel 

 with the same velocity under a given electric force ; 

 and further, behave identically with regard to their 

 power of acting as nuclei of condensation for super- 

 saturated water-vapour. It was therefore justifiable 

 to assume, in the first place, that the charge on an 

 ion produced by X-rays was equal to the charge on 

 an ion produced by the impact of ultra-violet light 

 on a metal surface. (This conclusion Prof. Thomson 





