92 ELECTRONS AND IONS [CH. ix 



electrons. The same value characterises even the 

 molecular aggregates which are often found in 

 conducting air or other gas. 



Fig. 14 shows H. A. Wilson's apparatus for sus- 

 pending a condensed cloud against gravity by means 

 of a known electric field. The magnet M, attracting 

 its keeper, opens connexion with an injector pump 

 through a valve V, and thereby, through the action 

 of the test tube T, produces a measured amount of 

 expansion in C. T. E. Wilson's manner as exhibited 

 in fig. 12. The expansion could subsequently be 

 measured by the gauge H. 



A cloud is formed in the bell-jar B, and the electric 

 field is applied between the plates attached to the 

 terminals C D, kept electrified by a battery of a great 

 number of cells. The battery, however, was only 

 applied after the cloud had formed, otherwise all the 

 ions would be removed from the vessel by electrolytic 

 action. Rontgen rays were used to produce the ions, 

 but the coil exciting the rays was switched off and 

 stopped, by means of an automatically working switch 

 S, the instant before the valve was pulled by the 

 magnet. 



J. J. Thomson has recently repeated his original 

 experiments, and by combining a determination of 

 New with an independent velocity measurement, he 

 has made what is now regarded as a standard estimate 

 of e, namely, 3'4 x 10~ 10 electrostatic units. 



H. A. Wilson's confirmation of this by a totally 

 different method the Mahomet's coffin method by 

 which he finds 3'1 x 10~ 10 , has already been mentioned. 



The importance of obtaining accuracy in these 

 measurements is that thereby lies our chance of 

 determining ra with accuracy the mass either of 

 an electron, or of an atom of matter. 



