﻿784: Dr. Norman Campbell on 



2. The liberation of electrons at metal surfaces under the 

 action of positive rays has been studied, under the name of 

 " secondary cathode radiation," by several investigators. 

 The only investigations which seem to throw any light on 

 the number of electrons which are liberated by each positive 

 ray are those of Fiichtbauer *, Baerwald f , and Koenigsberger 

 and Grallus J. Fiichtbauer found that when positive rays 

 (probably hydrogen) with an energy of from 31,300 to 

 15,000 volts fell on a copper plate, the number of electrons 

 liberated by each positive particle varied from 1*36 to 0*89, 

 while the proportion of the positive particles reflected varied 

 from 0*083 to 0*15. Baerwald, using positive rays of 

 hydrogen and helium falling on an aluminium plate, found 

 that the number of electrons liberated by each particle did 

 not change appreciably between 30000 volts and 900 volts, 

 but that the ionizing action ceased abruptly at this lower 

 limit. Koenigsberger and Grail as found that the number of 

 electrons liberated from brass by positively charged rays, of 

 which the nature and speed were not precisely determined, 

 lay between 2 and 4. 



Finally Bumstead §, working with a-rays, found that the 

 variation of the ionizing power of the rays at a metal surface 

 varied with the speed in almost exactly the same manner as 

 the ionizing power in a gas. Hauser || , in agreement with 

 other workers, finds the number of electrons liberated at a 

 metal surface by each a-ray to be about 60. 



3. In the experiments about to be described, which w T ere 

 conducted in an apparatus of which the' essential portion is 

 shown diagrammatically in fig. 1, the positive particles were 

 not (as in the work just mentioned) those of these canal-rays, 

 but those liberated from heated phosphates as in the work 

 of v. Bahr and Franck and of Pawlow. The use of such 

 particles is necessary if their effect is to be investigated for 

 very low potentials at which a discharge will not pass ; it 

 has the additional advantage, even' at higher potentials, that 

 the observations can be made in a very low vacuum and the 

 ionization completely localised at the metal surface ; it has 

 the disadvantage that greater uncertainty exists as to the 

 nature of the particles. The particles liberated at the heated 

 strip S, which was of platinum coated either with sodium 



* 0. Fiichtbauer, Phys. Zeit. vii. p. 153 (1906). 

 t H. Baerwald, Ann. d. Phys. xli. p. 643 (1913). 

 t J. Koenigsberger u. A. Gallus, Deutsch. Phys. Gesell. Verh. xvi. 4. 

 p. 190 (1914). 



§ H. A. Bumstead, Phil. Mag. xxii. p. 907 (1911). 

 H Hauser, Phys. Zeit. xii. p. 466 (1911). 



