[ 596 ] 



LXVIII. The Effect of an Electric Current on the 

 Photoelectric Effect. 

 To the Editors of the Philosophical Magazine . 

 Gentlemen, — 



IN the August number of the Philosophical Magazine 

 Dr. Frank Horton has criticised a paper of -mine on 

 the photo-electric emission from bismuth, published in the 

 June number. He ascribes my results to the emission of 

 gas by portions of the apparatus when slightly heated by the 

 current passed through the bismuth, and the subsequent pro- 

 duction of radiation and ionization in that gas by the photo- 

 electrons. 



I have definite experimental evidence against this ex- 

 planation. 



1. On one occasion a small heating-coil was placed beneath 

 the bismuth plate and was found to produce no change in 

 the photo-electric emission for the small rise of temperature 

 which it produced. This observation was not repeated and 

 is, therefore, given little weight. 



2. The early part of the experiment was carried out with 

 only — 4 volts instead of the — 14 volts which was used 

 later to obtain the saturation current. The rise in the photo- 

 electric current was just as evident at the low voltage as at 

 the high. 



3. The effect of the emission of gas should be just as 

 evident if the photo-electric current from the electrode is 

 measured. This control experiment was carried out with 

 both plates and films of bismuth by placing the plate at 

 4- 14 volts. Though very large currents were passed through 

 the bismuth, the only change in the current from the electrode 

 was a small decrease. If gas emission were the controlling 

 factor, this experiment would have shown rises corresponding 

 to those observed in the original experiment. 



-c, inn I am, Gentlemen, 



Emmanuel College, ' , r P ..\ P „ 



Cambridge, Yours faithfully, 



8th August, 1921. Allen G. Shenstone. 



in a 



* 



LXIX. Attempts to Detect the Presence of Neutrons 

 Discharge Tube. By J. L. Glasson, M.A. 



1. TNTRODUCTORY.-The possibility of the existence of 



X a substance of zero nuclear charge was first mooted 



by Sir Ernest Rutherford in the Bakerian Lecture for .1 920 



(Proc. Roy. Soc. vol. xcvii. A, p. 396). The existence of 



* Communicated by Prof. Sir E. Rutherford, F.R.S. 



