Contact Difference of Potential of Distilled Metals. 337 



Rutherford and Robinson * have found a few /3 particles 

 from radium C carrying as much as 3*9 x 10" 6 erg. 



Similarly the fastest {3 ray observed from radium B has an 

 energy of *6 x 10 ~ 6 erg, while a very rough approximation 

 to the combined energy of the (3 and y radiations gives 

 •7xl0" 5 erg. 



The results in the case of radium are thus at variance 

 with the theory, but the discrepancy is hardly greater than 

 can be explained by the uncertainty in the exact value of the 

 heat produced by the y radiation, and in other factors in 

 the calculations. 



We wish to thank Sir Ernest Rutherford, who suggested 

 the work, for his advice and interest during its progress. 



Physical Laboratory, 



University of Manchester. 



XL. The Contact Difference of Potential of Distilled Metals. 

 By A. Ll. Hughes, B.A., D.Sc, Assistant Professor of 

 Physics, The Rice Institute, Houston, lexas'f. 



THIS paper contains a short account of some experiments 

 on the contact difference of potential between metals 

 and particularly on its relation to the presence or absence 

 of occluded gases. ' The research was suggested by a 

 comparison of the photo-electric experiments of Richardson 

 and Compton with those carried out by the writer. A 

 discussion of the relations between photo-electricity and 

 contact difference of potential will be given before the actual 

 experiments are described. 



One of the most important problems in photo-electricity is 

 the determination of the exact relation between the maximum 

 emission velocity of the photo-electrons and the frequency 

 of the light causing their emission. Experimental diffi- 

 culties made it impossible, until recently, to determine the 

 exact way in which the velocity increased with the frequency. 

 The relation between the two quantities has been shown by 

 Richardson and Compton J, and by the writer § to be that 

 the velocity squared, or the energy, of the fastest photo- 

 electrons is a linear function of the frequency. This is 

 generally expressed in the form 



V = kn-V , (1) 



where V is the potential just necessary to stop the electron 



* Rutherford and Robinson, Phil. Mag-, xxvi. p. 717 (191,3). 

 f Communicated by the Author. 



X Richardson and Compton, Phil. Mag. xxiv. p. 575 (1912). 

 § Hughes, Phil. Trans. A. ccxii. p 3 205 (1912). 

 Phil. Mag. S. 6. Vol. 2$. No. 165. Sept. 1914. Z 



