and a Theory of Contact-Potential. 327 



That metal to which the electro- \ t!e active ™ n & oes > rece i~ 



vin£ an electro- \ P 0S1 *T e charge, acquires thereby an elec- 

 *V .,. J negative * ; u J 



I "Dositive 

 tro- "J . f • , potential relatively to that of the electrolyte. 



Thus, when iron and copper are each of them placed alone 

 in water, both metals are oxidized, though differently: both 

 therefore are negative, though by different amounts, to the 

 water; but if these metals are placed in the same water, the 

 iron alone is oxidized, the copper has hydrogen deposited on 

 it and becomes therefore positive to the iron, as before, but in 

 addition is positive to the acid. 



We have as yet no means of estimating the actual difference 

 of potential between an electrolyte and any metal; for the 

 measurement of the Peltier effect on the passage of a current 

 is the only way of determining this difference, and such a 

 method is impracticable. Since there exists no proof of the 

 contrary, I have assumed that when two metals are introduced 

 into an electrolyte, the potential of the latter is intermediate 

 between those of the two metals. Now, if the production of 

 potential depends on the charges brought by the dissociating 

 ions, this assumption must necessarily represent the actual 

 case ; and it is moreover borne out in the experiments that 

 follow. 



We have therefore two cases to consider, whether in liquids 

 or in gases : — The metals (say iron and copper) are in the same 

 electrolyte and 



(1) Insulated. In this p^ ^ 

 case the iron is negatively D ' cu 

 charged by the electro- Air or 

 negative ion, the copper 

 is coated with hydrogen 

 (in the case of liquid elec- 

 trolytes, and also in that £l 



of gases, if the distance 

 between the metals is suf- 

 ficiently small to overcome in part electrolytic diffusion), and 

 thus is at a positive potential relatively to the electrolyte. 



(2) Touching one another. 

 The metals are then at the Fig. 3. 



same potential ; but the same 

 differences of potential be- 

 tween metal and electrolyte 

 must obtain, and this is only 

 possible when the potential 

 around the iron is higher 

 than around the copper. 



2C2 



