Measurement of Electrode-Potentials in Liquids. 25 



potentiometer is that between the cathode and the standard 

 electrode, and this is composed of the following parts : — 



(1) The drop of electromotive force due to polarization 

 between the cathode and the layer of liquid in contact 

 with it. 



(2) The difference of potential between the cathode and T 

 calculated according to Ohm's law r as the product of the 

 resistance between the two and the current strength. 



(3) Any small electromotive force brought about by con- 

 tact of the electrolyte examined with the decinormal potassium 

 chloride solution of the standard electrode. 



(4) The constant E.M.F. of the standard electrode. 



The first of these quantities is the one which it is desired 

 to measure, the third was always assumed to be zero, as has 

 been done by other investigators. The fourth may be taken 

 according to Ostwald and Wilsmore to be 0*615 volt for the 

 standard so-called decinormal electrode composed of mercury 

 in contact with a decinormal solution of potassium chloride 

 saturated with mercurous chloride. Being constant for all 

 experiments it needs, however, not be taken into considera- 

 tion. The second difference of potential mentioned, may as 

 a rule be calculated when the specific resistance of the elec- 

 trolyte is known. It is better, however, actually to determine 

 the resistance between the cathode and T during the course 

 of an experiment, more especially when there is a possibility 

 of an increase of resistance at the electrode, owing to the 

 formation of badly conducting deposits, gas-bubbles, &c. 

 The electrical method required for this purpose had to be 

 devised in such a manner that no false connexions should be 

 introduced and that it should at the same time be independent 

 of the polarization of the electrode. As far as I am aware, 

 the only method suitable for this purpose is one described 

 several years ago by Nernst and Haagn*. It consists of a 

 modification of the Wheatstone-bridge method for alternating 

 currents, two resistances being replaced by standard con- 

 densers, and rough condensers being interposed wherever a 

 false connexion would otherwise be made. Its application to 

 the present case is shown diagrammatically in fig. 3. The 

 two resistances compared are the resistance from the anode 

 to T, and the resistance from T to the cathode, plus a known 

 variable resistance M. These formed two arms of the Wheat- 

 stone bridge, the other two being the standard condensers A 

 and B, fig. 3. A special box was made up for this determina- 

 tion containing capacities varying from one-thousandth of a 

 microfarad to half a microfarad. In order to form the bridge 



* Zeitschft. l J hys. Chem. xxiii. p. 97. 



