278 THE ELECTRIC TELEGRAPH. 



37. Determination of Gfatranic Polarisation. Tlie deter- 

 mination of the resistances of fluids cannot always be made 

 by the direct substitution of a metallic resistance giving the 

 same deflection of the needle of the measuring instrument, 

 because the electro-motive force in the circuit of a fluid 

 resistance is not always that of the cell or battery by which 

 the resistance is measured, but generally this minus the 

 electro -motive force of a polarising layer of gas forming on 

 the plates or electrodes immersed in the fluid. 



In a circuit, therefore, containing a metal resistance, R, 

 and the resistance of a fluid column, r, the current, I, 

 measured by the galvanometer, is 



< 



E being the electro-motive force of the measuring battery, 

 and e that of the polarisation of the plates with the reverse 

 sign. The truth of this employment of the law of Ohm 

 may be proved by varying the two members of the denomi- 

 nator : R, by varying the length of wire, and r by varying 

 the distance between the electrodes. Whereas, when the 

 fluid conductor is removed, and a metallic resistance (=>) of 

 the same value introduced in its stead, the deflection indi- 

 cates another current, say I', and we have 



" 



To find now the value of e, we put so much extra resistance, 

 r, into circuit Number 1, that the needle's deflection is 

 sensibly diminished, indicating a current of some inferior 

 strength, I L , expressed by 



I = E "" * (II 



R + r + r' 



Equations (I. and (II. combined give the value of the difference, 

 E e, between the two opposite electro-motive forces, by an 

 expression from which the unknown resistances, II and r, of 

 the wire and fluid columns are eliminated : - 



E-^r ..... (III. 



