286 Prof. J. A. Fleming : Note on the Photoelectric 



o 



the presence of helium was detected in a cylinder of tinfoil 

 exposed for a few hours over the emanation-tube. 



These experiments show that the helium does not escape 

 at once from the lead, but there is on the average a period 

 of retardation of several hours and possibly longer. 



The detection of helium in the lead and tin foil, as well as 

 in the glass, removes a possible objection that the helium 

 might have been in some way present in the glass initially, 

 and was liberated as a consequence of its bombardment by 

 the ol particles. 



The use of such thin glass tubes containing emanation 

 affords a simple and convenient method of examining the 

 effect on substances of an intense a radiation quite inde- 

 pendently of the radioactive material contained in the tube. 



We can conclude with certainty from these experiments 

 that the a particle after losing its charge is a helium atom. 

 Other evidence indicates that the charge is twice the unit 

 charge carried by the hydrogen atom set free in the electrolysis 

 of water. 



University of Manchester, 

 Nov."l3, 1908. 



XXII. A Note on the Photoelectric Properties of Potassium- 

 Sodium Alloy. By J. A. Fleming, M.A., JD.Sc, F.R.S., 

 Professor of Electrical Engineering in University College, 

 London *. 



IX connexion with further researches on my oscillation 

 valve or glow-lamp radiotelegraphic wave-detector, I 

 was led to examine instances of electronic emission other 

 than that due to the incandescence of metals or carbon in 

 vacuo, to ascertain how far rectifying effects with high 

 frequency currents could be obtained by theinj. It is well 

 known that under the action of ordinary and ultra- 

 violet light the electropositive metals lose a negative charge 

 of electricity, and it was shown by Elster and Geitel that 

 this photoelectric effect is most pronounced in the case 

 of rubidium, potassium, and the liquid alloy of potassium and 

 sodium. 



With the object of examining this effect, experiments were 



* Communicated by the Physical Society : read November 13th, 1908. 



t See J. A. Fleming-, " On the Conversion of Electric Oscillations into 

 Continuous Currents by means of a Vacuum Valve," Proc. Roy. See. Lond. 

 vol. Ixxiv. p. 476 (1905). Also " The Construction and Use of Oscillation 

 Valves for rectifying High Frequency Currents/' Phil. Mag. May 1900, 

 p. 659. 



