The essence of the law is the proportionality between C
and E, which means that the ratio E/C is a constant. But E/C = R, and
thus the law may be tested by examining the constancy of the measured
resistance of a conductor when different currents are passing through
it. In this way Ohm's law has been confirmed in the case of metallic
conduction to a very high degree of accuracy. A similar principle was
applied by Kohlrausch to the case of electrolytes, and he was the first
to show that an electrolyte possesses a definite resistance which has a
constant value when measured with different currents and by different
experimental methods.

_Measurement of the Resistance of Electrolytes._--There are two effects
of the passage of an electric current which prevent the possibility of
measuring electrolytic resistance by the ordinary methods with the
direct currents which are used in the case of metals. The products of
the chemical decomposition of the electrolyte appear at the electrodes
and set up the opposing electromotive force of polarization, and unequal
dilution of the solution may occur in the neighbourhood of the two
electrodes. The chemical and electrolytic aspects of these phenomena are
treated in the article ELECTROLYSIS, but from our present point of view
also it is evident that they are again of fundamental importance. The
polarization at the surface of the electrodes will set up an opposing
electromotive force, and the unequal dilution of the solution will turn
the electrolyte into a concentration cell and produce a subsidiary
electromotive force either in the same direction as that applied or in
the reverse according as the anode or the cathode solution becomes the
more dilute. Both effects thus involve internal electromotive forces,
and prevent the application of Ohm's law to the electrolytic cell as a
whole. But the existence of a definite measurable resistance as a
characteristic property of the system depends on the conformity of the
system to Ohm's law, and it is therefore necessary to eliminate both
these effects before attempting to measure the resistance.

The usual and most satisfactory method of measuring the resistance of
electrolytes consists in eliminating the effects of polarization by the
use of alternating currents, that is, currents that are reversed in
direction many times a second.[11] The chemical action produced by the
first current is thus reversed by the second current in the opposite
direc