SECTION III., 1882. ee alle | Trans. Roy. Soc. CANADA. 
III.— On the Measurement of the Resistance of Electrolytes by Means of Wheatstone’s 
Bridge. By Proressor J. G. MacGrecor, M.A., D.Sc., FR.S.E. 
(Read May 26, 1882.) 
The chief difficulty in the measurement of the resistance of electrolytes is due to the 
polarization by the current of the electrodes of the electrolytic cell. The electromotive 
force which is thus produced in the very process of measurement renders it impossible to 
use the Wheatstone’s bridge in the simple manner in which it is used to measure the 
resistance of metallic conductors. Special precautions must be taken either to reduce or 
to remove the polarization or to enable the measurement to be made in spite of it. The 
bridge method of Kohlrausch, Nippoldt, and Grotrian* was based upon the reduction of 
polarization, which was secured by the use of alternating currents and of electrodes haying 
a large surface. In consequence of their use of alternating currents, they were forced to 
employ as galvanoscope Weber's electrodynamometer, an instrument which is much more 
sluggish than the galvanometer. Prof. J. A. Ewing and myself found it possible to make 
very approximate determinations of the resistance of electrolytes by means of the bridge, 
without any attempt to reduce the polarization, but by simply using as galvanoscope a 
galvanometer with a magnet of very small moment of inertia. Hach of these methods has 
its own advantages. That described in this paper combines in itself the advantages of 
both. It involves the employment of a delicate galyanometer (Sir Wm. Thomson’s “ Dead 
Beat,” with very light mirror) in the bridge, the reduction of polarization after the manner 
of Kohlrausch, and the elimination of the polarization thus reduced and of other sources of 
error by the use of electrolytic cells in two arms of the bridge. 
I at first used, as the source of alternating currents, a magneto-electric machine, con- 
sisting of a coil of wire within which a magnet was made to revolve. I need not give a 
more accurate description of the instrument, or state its dimensions, as I was compelled, 
after making a series of experiments with it, to adopt an entirely different mode of pro- 
ducing currents. Alternating currents, however, from whatever source, could produce 
only a trembling motion of the magnet of a galvanometer, and with a rapid alternation 
such as I wished to use, a trembling motion would not be capable of observation. Hence, 
since I wished to use a galvanometer as a galvanoscope, the currents in the galvanometer 
branch of the bridge must be made to flow continually in the same direction. For this 
purpose, according to a suggestion made by Prof. Wiedemann, of Leipzig, I used a com- 
mutator such as is employed to give constancy of direction to the currents from an induc- 
tion machine. It was fixed to the axis of revolution of the magnet, and so placed that the 
greatest current was produced when the magnet was made to rotate at a certain fixed rate. 
The ends of the coil of the induction machine dipped in mercury cups at opposite angles 
of the Wheatstone’s bridge. From the other angles, wires went to the commutator with 
which the galyanometer also was joined. By this arrangement, while alternating currents 

* Pogg, Ann, CXX XVIII, (1869), pp. 280,370 ; CLIV, (1875), p. 1. 
