458 



Prof. 0. Lodge on the Controversy 



his calculation theoretically to have, an E.M.F. of *51 or *52 

 volt. 



Ag | 0-1 AgN0 3 I KN0 3 I 1-0 KC1 I AgCl j Ag, 



the KNO3 being introduced for obvious reasons to prevent 

 precipitation of adjacent solutions, but being without influence 

 on the resultant E.M.F. 



To calculate the total E.M.F. of a cell, however, it is 

 necessary to take the other boundaries of the liquid into account. 

 It is useless to complete the circuit by liquid, it must be com- 

 pleted by metals, and in the cases now being considered by 

 one and the same metal. Each metal acts as a wall pre- 

 venting effective diffusion of one at least of the liquid ions : 

 the cathode prevents the cation from advancing, the anode 

 prevents the anion ; whatever else the metals may do will be the 

 same at each end and so balance. This is I believe the form 

 given by Mr. Whetham to Prof. Nernst's argument. 



Hence the total E.M.F of such a cell will be the sum of the 

 three junction-E.M.F.'s, metal/liquid, liquid/liquid, liquid/ 

 metal : 



\u x + o 6 



2v 1 



+ — -log— 



u i + v i n 



+ - — — logn' J 



= ^RT 



u l -j-v 1 °^ri 



There is here, fortunately, no question about an innocent- 

 looking but controversial metal/metal junction. 



The remarkable thing about this equation is that the data 

 on the right-hand side are all known, and that therefore E can 

 be calculated in absolute measure. 



Now since the characteristic gas constant R, the absolute 

 specific heat of expansion per gram of perfect gas, varies 

 inversely with m the atomic weight, it follows that Rm is an 

 absolute constant, the same for every substance, or at least 

 only differing by a simple multiple, having twice the value 

 for electrolytic as for undissociated molecular substances ; 

 hence we may take its value as double that for, say, hydrogen. 

 For hydrogen m/e = '00010352 gramme per coulomb, and 

 R= 4:1*6 x 10 6 ergs per gramme degree. 



Hence at 15° C. 



E = 2x4'16 x -00010352 x 2&8 — : — log e -Joules per coulomb 



u + v 



•114 1 n . 



r^ loglo n' volts > 



where r is written for the Hittorfian migration ratio of anion 



