394 COMPARISON OF RESISTANCES. 



Two insulated platinum wires, called parasite electrodes, dip two 

 points C and D of the liquid, and communicate with the elec- 

 trodes of an electrometer. The deflection measures the difference 

 of potential of the two points C and D. This difference is equal 

 to the product IR, R being the resistance of the liquid column 

 between the two planes C and D if the wires are not polarized. 

 It is sufficient for this that they have not given passage to an 

 appreciable current, which amounts to saying that the capacity of 

 the electrometer is infinitely small compared with the capacity of 

 polarization of the wire. 



The capacity of a quadrant electrometer is always very small 

 in comparison with that of parasite electrodes. With a capillary 

 electrometer care should be taken that the immersed surface of 

 the electrodes is very great compared with that of the mercury 

 in the capillary tube of the electrometer. In this case it is ad- 

 vantageous to use platinised plates of platinum, the capacity of 

 polarization of which is usually 20 to 25 times greater than ordi- 

 nary platinum. 



In order to compare directly the resistance of the liquid column 

 with that of a metal conductor, the principal current is made to 

 pass through a resistance box, and the two parasite electrodes of 

 the liquid and those of the two points of this box are alternately 

 connected with the electrometer. If the indication is the same 

 in both cases, the resistances are equal. If there is an accidental 

 difference between the electromotive forces of contact at C and D 

 independent of the current and of the polarization, it is eliminated 

 by reversing the direction of the current and taking the mean of 

 the deflections observed. 



992. USE OF SINUSOIDAL CURRENTS. In measuring liquid 

 resistances, Professor Kohlrausch* used a curious property of alter- 

 nating currents sinusoidal in form. 



Let us consider a circuit containing a voltameter and a source S. 

 If R and L are the resistance and the coefficient of self-induction 

 of the circuit, E the electromotive force of the source, e the elec- 

 tromotive force of polarization of the voltameter, we have for any 

 given epoch 



(48) L 



* F. KOHLRAUSCH. Fogg. Annalen, Vol. cxxxvm., pp. 280 and 370, 1869; 

 Vol. CXLVIII., p. 143, 1873; Jubelband, p. 290, 1874. 



