244 J. M . Lewis: 



If an equivalent weight of an univalent metal goes into solu- 

 tion, the charge carried by the ions will be 96540 coulombs, i.e., 

 the Faraday constant F. However, total electricity is equal to 

 quantity multiplied by intensity or potential; the formula will 

 therefore bey P = RT } n P/p or P = RT/F \n Yjp. Where the metal is not 

 univalent, but has the valence n, this factor is introduced thus: 

 P = RT/»F1«P//. (8). Now R, in electrical units, is 8.3 joules per 

 degree, and F, in coulombs, 96540, while the absolute tempera- 

 ture T, is 291 (273 plus 18). The formula RT/F thus becomes: 



8-3x291x2-3 " 



— nc „ Ar . = -058z> 



96o40 



The modulus 2.3 is introduced in order to bring natural to 

 common logarithms. (9) 



The significance of this equation is that a ten-fold change in the 

 concentration would cause a difference of 0.058 v. in the elec- 

 trode potential for a univalent ion, 0.029 for a bivalent, and 0.193 

 for a tervalent, and it certainly holds good for solution below 

 decinormal strength. (10) The practical application of this will 

 be considered later. 



The e.m.f. developed by the hydrogen electrode varies within 

 fairly "narrow limits, and is never greater than a fraction over 

 one volt. For the measurement of such currents where extreme 

 accuracy is not called for, as in determining the end point of 

 a reaction, or checking the normality of solutions used in volu- 

 metric analysis, a sensitive galvanometer may be used as described 

 by Hildebrandt. (11) 



In biochemical work, however, the standard practice is to em- 

 ploy the Poggendorf compensation method, using a potentiometer 

 for this purpose. 



In its simplest form (shown diagramatically in Fig. 5), the ap- 

 paratus consists of a wire, a, b, of uniform calibre, stretched 

 along a wooden " metre stick." To this wire a steady e.m.f. of 

 about 1.5 volt is applied by means of an accumulator C. If now 

 a cell D, whose e.m.f. it is desired to measure, be so connected 

 to ab, that its e.m.f. is opposed to that of the cell C, and is sup- 

 plied with a movable contact which slides along ab, then a point 

 may be found where no deflection takes place in the galvanometer 

 E,' which is inserted in the circuit. .... 



When this point is found, then the e.m.f. of the cell D is equal 

 to the potential drop along AF, and it therefore corresponds to 

 the ratio AF.FB. 



