382 MEASUREMENT OF THE RESISTANCE 



MacGregor in the paper already referred to, I need give but a 

 short sketch of it here. The electrolytic conductor is introduced 

 by Platinum electrodes as one of the arms of Wheatstone's 

 Bridge, and the process of determining its resistance is the same 

 as in the case of a metallic conductor, viz., by a gradual adjust- 

 ment of the arms. The indications of the Galvanometer, how- 

 ever, will not be the same as in that case, owing to the fact that 

 the electrodes become polarized through the passage of the 

 current. The effect of this polarization on the Galvanometer is 

 the same as if, during the passage of the current, the resistance 

 of the electrolytic cell gradually increased. 



Imagine the arms of the bridge so adjusted that, on the pas- 

 sage of the current, the light spot of the Galvanometer moves off 

 to the right and remains there, such a deflection having been as- 

 certained to mean that the resistance being measured, if metal- 

 lic, was too small to give zero deflection. If we gradually change 

 the adjustment of the arms in such a way as to diminish this 

 deflection to the right, it will not be found possible to obtain 

 an adjustment making the deflection zero, but an adjustment will 

 be reached with which the light spot moves first to the right, 

 stops, moves back and off' to the left, showing that though at the 

 beginning of the flow of the current, the resistance of the cell was 

 small enough to give a deflection to the right, it has been vir- 

 tually increased by the polarization until it is so large as to give 

 a deflection to the left. We have thus a double deflection. As 

 we continue changing the adjustment so as to diminish the 

 initial deflection to the right, the double deflection will grow 

 less and less until finally we shall have only the deflection to 

 the left. 



If we had a magnet and mirror of indefinitely small moment 

 of inertia, hung by a fibre offering no resistance to torsion, so that 

 it would, at any instant, indicate the direction of the current 

 flowing through it, this double deflection would only just vanish 

 when the adjustment of the arms indicated the exact resistance 

 of the cell, i. e., when the resistance of the cell, calculated in the 

 ordinary way from the resistances in the other arms, would be 

 its exact resistance. 



