306 On Contact Difference of Potential. 



wires N. X is the source of light, a quartz-glass mercury 

 vapour lamp. Inside the cylinder was placed a shallow 

 paraffin dish containing uranium oxide. If everything is at 

 first earthed, and then the quadrants in connexion with Z 



Fijr. 1. 



TO ElECTROM£T£,9 



T.O E/IRTH OR 

 fo TENT/OMET£fi. 



insulated, the deflexion of the electrometer-needle will give- 

 the contact difference of potential of the zinc plate and 

 copper vessel. The object of the experiment is to find the 

 change in the deflexion produced by the ultra-violet light. 

 It must be observed that there are two effects on the 

 needle caused by the action of the light. The first is the 

 loss of electrons by Z, which means the communication to 

 it of a positive charge. The second is the increase or 

 decrease of the contact difference of potential which results 

 in the communication of a negative or positive charge to Z. 



On account of these interfering effects, it was found more 

 satisfactory to first expose the zinc plate for a certain time to 

 the source, and then immediately afterwards to measure the 

 contact difference. It must be remembered that, as one face 

 only of the zinc plate was exposed, and as the other face was 

 not cleaned, the result has no importance from a quantitative 

 point of view. In one experiment the zinc plate was exposed 

 for 30 minutes. The contact-difference before exposure was 

 •45 volt, after exposure it was '21 volt. It then gradually 

 rose and nearly attained its initial value in nine days, after 

 which it fell very slowly. It thus appears that in the case 

 of zinc the effect, although lasting for a very appreciable 

 time, is temporary. An experiment, in which a plate of 

 aluminium was substituted for the zinc, showed a fall from 

 1*01 volt to "37 volt, with a gradual rise in three days to 

 *48 volt, after which it remained practically steady for 

 several days, and was not measured afterwards. No doubt 



