252 Mr. Norman Shaw on Charged Surface Layers in 



becomes tarnished or dirty it always assumes a potential 

 nearer that of its surroundings.] 



(2) It was found that for different metals tested the 

 charging curves appeared to differ merely in proportion to 

 the contact potential. Thus the curve obtained by first 

 charging up the plate C to some potential P x + X where P x 

 is the contact potential between the plate C and the brass, 

 and then insulating, was similar to that obtained when 

 another plate was substituted for 0, charged up to P 2 + X 

 and insulated, P 2 being the contact potential between the 

 second plate used as and the brass. In Table IV. a com- 

 parison is shown between curves of the same type for zinc- 

 brass and brass-brass, in which the plate C in the latter case 

 has been charged to a potential Pi — P 2 , where V 1 is the 

 contact potential between the two brass plates and P 2 that 

 between the zinc and brass. In the case of the experiment 

 recorded, P 1= 0-060 volt and P 2 =0'546 volt. 



Table IV. 



Time. 



Zinc-Brass. 



Brass-Brass. 



sec. 



0000 



0-000-0-486 



15 



0-229 



0231- „ 



30 



0-363 



0366- „ 



45 



0-448 



0-444- „ 



lm. 



0-491 



0-489- „ 



15 



0519 



0-518- „ 



30 



0530 



0-530- „ 



45 



0-540 



0-541- „ 



2 m. 



0541 



0-542- „ 



In fig. 4, curve (3) represents these sets ; the zinc-brass 

 points are represented by crosses, and the brass-brass points 

 by small circles, the value P x — P 2 ( — 0*486 in this case) being 

 omitted in the plotting in order to facilitate comparison. It 

 will thus be observed that the rate of charging (or removal) 

 of layers is independent of the metal and the manner in 

 which the layers are produced. The general behaviour is 

 the same whether the layers are produced by the phenomenon 

 of contact potential or by external means, and the behaviour 

 of the layers on one metal can be made similar to those on 

 another by supplying the difference in surface charge by 

 external means. This similarity in all metals appears to be 

 further evidence in favour of the idea that contact potential 

 phenomena are due to some intrinsic property of metals in 

 which the boundary conditions vary for different metals, 

 such as, for example, in the conception that it depends upon 



