1917] Industrial Applications of Electrons 175 



WEEKLY EVENING MEETING, 



Friday, June 8, 1917. 



The Hox. Sir Charles Parsoxs, K.C.B. J.P. Sc.D. LL.D. F.R.S., 

 Yice-Presiclent, in the Chair. 



Professor Sir J. J. Thomson, O.M. LL.D. D.Sc. Pres.R.S. 

 Professor of Natural Philosophy, R.I., Master of Trinity. 



Industrial Applications of Electrons. 



If there are among my audience any who twenty years ago listened 

 to the announcement I made here of the existence of electrons, they 

 will, I think, admit that they would admit that they would have 

 been very sceptical if they had been told they would in another 

 twenty years be listening to another discourse on the commercial 

 applications of these electrons. For electrons are so small that it 

 takes about 1700 of them to give a mass equal to that of an atom of 

 hydrogen, and they move at such a rate that they get out of the 

 field of operations in a small fraction of a millionth of a second. 

 Such properties appear rather transcendental, and not promising from 

 a practical point of view. But, as a matter of fact, electrons were 

 now not only in trade circulars, but also in the I^aw Courts. 



Yet electrons possessed properties which gave great promise for 

 useful application. Inventors would appreciate the importance of 

 having a body with practically no mass, and yet capable of being 

 acted upon by forces. Again, the electrons existed for exceedingly 

 brief periods, and were independent of what had happened before, or 

 what might happen afterwards. Thus they gave really instantaneous 

 photographs, and not averages over periods comparable with even 

 thousandths of a second. For particles existing only for a hundredth- 

 millionth of a second, the period of an electric wave was almost an 

 eternity, and the particles would register the periodical forces in such 

 waves as readily as if they were steady. This may, I think, lead to 

 a convenient method of registering electric waves. Let the stream 

 of cathode particles pass through the electric field between two 

 parallel plates and connect these plates to an antenna. "When no 

 waves were coming along, the spot of light marking the place where 

 the cathode rays struck the phosphorescent screen would remain 

 fixed, but when the waves came, it would be drawn out into a con- 

 tinuous line. It was easy to see that if the incident wave were cut 

 up into a series of dots and dashes it would affect the nature of the 

 trace made by the cathode rays. 



The use of electrons in " hot wire valves " to detect electrical 



