concerning 



Valla's Contact Force. 357 



turbing conditions that the difference of potentials between 

 two metals in contact varies with the temperature of the 

 junction. 5 ' [This is no doubt true as it stands, if the junction 

 is the only part heated ; the Seebeck force is superposed 

 upon the Volta force, and hence the electrostatic effect obser- 

 vable in an incomplete circuit is naturally slightly affected 

 by differences of temperature, though not necessarily by 

 absolute uniform temperature. The break of circuit may be 

 made in the middle of one of the metals whose junctions are 

 maintained at different temperatures, in which case only the 

 Seebeck force could possibly be electrostatically observed 

 (that is, a combination of Peltier and Thomson effects) by a 

 sufficiently sensitive electrometer ; or the break may be 

 made between the two metals, in which case the much 

 greater Volta force will be observed in the air-gap in com- 

 bination with the insignificant Peltier force of the metallic 

 junction ; and any fluctuation of temperature to which that 

 remaining metallic junction is subjected will but vary this 

 insignificant superposition upon the true Volta effect. 

 But the sentence quoted is not intended to convey this 

 meaning ; what it is intended to convey is that the Volta 

 gradient of potential observed in the gap is chiefly due to 

 something occurring at the remaining metallic junction, and 

 that this something is a function of the temperature. Indeed, 

 a later sentence (§ 26) clearly states, not only the existence 

 of this large metallic-junction force, but also its cause.] 

 " This force " [viz. the force of attraction between the 

 oppositely-charged metals across a gap] , " properly viewed, 

 is a resultant of chemical affinity between thin surface-layers 

 of the two metals." 



[And then the author goes on to explain that the junction- 

 force at the boundary of tw r o metals has nothing whatever to 

 do with any reversible heat-effects which may be observed 

 there ; because, if it had, these reversible heat-effects would 

 be vastly larger than in fact they are. It is therefore argued 

 that these Peltier effects by no means represent the value of 

 the E.M.F. existing at the junction where they occur, but 

 represent only the rate at which this E.M F, varies with 

 temperature, II = lVE/dT ; whereas, as I have said above, 

 the E really appropriate to this equation is not located at the 

 junction at all, but is the resultant or integrated' E.M.F. of 

 the whole circuit. However, I continue the quotations, not 

 at first using quotation marks simply because the wording is 

 abbreviated where no precision is required.] 



Imagine a circuit of two metals — say iron and copper; and 



