106 LORENTZ— ON POSITIVE AND NEGATIVE ELECTRONS [April 17 



metals M and M', say from the former towards the latter. Consid- 

 ering two sections, 6^ and S\ of the two metals quite near their sur- 

 face of separation, I shall denote by w^ the number of positive par- 

 ticles traveling across vS per unit of time in the nominal direction of 

 the current, by iio that of the negative ones going the other way, and 

 by w/, 112' the corresponding numbers for the section S\ Then, if 

 for the sake of simplicity we suppose all electrons to have equal 

 charges, we shall have 



^h + ^^2 = ^h' + ^h', 



but of course this does not imply that ^i and n.. are separately 

 equal to n^' and Ho'. If the Hall-effect is not the same in the two 

 substances, the ratio between ii^^ and iio will be different from that 

 between ;j/ and n^ ; it may very well be that n^ is much larger 

 than Uo and «/ much smaller than 11.2'. 



In order to fix our ideas, I shall suppose 



This means that the number of positive electrons entering the 

 space between vS' and S' through the first of these sections exceeds 

 the number leaving it through the second, so that the number con- 

 tained within the space will increase by n^ — n^\ As there will be 

 an equal increase ^2' — n^ of the number of negative particles the 

 result is a continual accumulation at the junction itself of equal 

 positive and negative charges, or, as we may say, of neutral elec- 

 tricity. Conversely, neutral electricity would continually be carried 

 away from the place of contact, if the direction of the current were 

 reversed. 



It is further to be noticed that a change in the distribution of 

 neutral electricity would even occur if we had no current at all ; if 

 there were two kinds of movable electrons, it would already arise 

 from the causes which produce the phenomenon of contact elec- 

 tricity. As to these causes several hypotheses have been put forth, 

 of which two may be briefly mentioned. In his celebrated paper on 

 the conservation of energy, Helmholtz accounted for the difference 

 of potential between two metals by means of certain attractive forces 

 exerted at very small distances by the material atoms on the particles 



