382 Prof. 0. Lodge on the Controversy 



number of atoms per square centimetre in the layer facing the 

 zinc will therefore be of the order 10 16 . Of these only a portion 

 will be oxygen ; and probably it may be said that only a small 

 proportion are ionised and so open to combination. But as 

 I do not need combination, I see no necessity to postulate 

 ionisation, or to care what proportion are in this condition. 

 Certainly I need molecules capable of being polarised 

 electrically, but that at the present date will hardly be denied 

 to any material molecules whatever. 



Very well then, if for any reason polarisation takes place, 

 the surface-density of the charge on either face of the double 

 layer can be 10 16 x 10 -1 1 = o~ = 10 5 electrostatic units per square 

 centimetre, at a maximum. The mechanical tension across the 

 layer is 2tto- 2 , and the intensity of electric field there is 47r<7, 

 which is also the gradient of electric potential. 



Now the thickness of the layer is of molecular dimensions. 

 10 -8 centim. say; hence the difference of potential between 

 its faces, that is the difference of potential between the zinc 

 and the air in its neighbourhood, is kiraz, which is of the order 

 (12 x 10 5 x 10~ 8 = TO -2 ), one-hundredth of an electrostatic 

 unit, or 3 volts. Now plainly this is of the right order of 

 magnitude. The Yolta effect observed can be explained by 

 such an electrical double layer, by such a chemical layer of 

 straining oxygen atoms, at a metallic surface *. 



That it can be equally well explained for metals in absolute 

 vacuum or free sether I would not presume to deny ; but I 

 cannot do the arithmetic for that case, because of an utter lack 

 of data. It is to be observed, however, that the only gaseous 

 or material substance needed (other than metal) is the coherent 

 film on the surface ; all the rest of the gas is merely a dielectric 

 medium for the transmission of electrostatic induction or 

 lines of force, and for this function an absolute vacuum serves 

 perfectly. Consequently, merely removing the plates into 

 the receiver of an air-pump will make no practical difference. 



But still, so far, I have not met the hypothetically suggested 

 difficulty about the condenser of large capacity and the amount 

 of real chemical action needed to account for the current 

 required to charge it to the voltaic difference of potential. 



We have only to proceed with the calculation. 



The condenser formed by the opposing surfaces of zinc and 

 copper is a condenser of the same (or comparable) area to 

 that molecular condenser just postulated between either metal 

 and the air. The only difference is that these last are of 

 molecular thinness, whereas the condenser formed by welL- 



: * See also a footnote to p. 379 of a paper in the Phil. Mag. for October 

 1885/ where I reckoned practically the same thing-. 



