en determined by
Kohlrausch by comparison with mercury, and, by using one of these
solutions in any cell, the constant of that cell may be found once for
all. From the observed resistance of any given solution in the cell the
resistance of a centimetre cube--the so-called specific resistance--may
be calculated. The reciprocal of this, or the conductivity, is a more
generally useful constant; it is conveniently expressed in terms of a
unit equal to the reciprocal of an ohm. Thus Kohlrausch found that a
solution of potassium chloride, containing one-tenth of a gram
equivalent (7.46 grams) per litre, has at 18 deg. C. a specific
resistance of 89.37 ohms per centimetre cube, or a conductivity of 1.119
X 10^-2 mhos or 1.119 X 10^-11 C.G.S. units. As the temperature
variation of conductivity is large, usually about 2% per degree, it is
necessary to place the resistance cell in a paraffin or water bath, and
to observe its temperature with some accuracy.

Another way of eliminating the effects of polarization and of dilution
has been used by W. Stroud and J. B. Henderson (_Phil. Mag._, 1897 [5],
43, p. 19). Two of the arms of a Wheatstone's bridge are composed of
narrow tubes filled with the solution, the tubes being of equal diameter
but of different length. The other two arms are made of coils of wire of
equal resistance, and metallic resistance is added to the shorter tube
till the bridge is balanced. Direct currents of somewhat high
electromotive force are used to work the bridge. Equal currents then
flow through the two tubes; the effects of polarization and dilution
must be the same in each, and the resistance added to the shorter tube
must be equal to the resistance of a column of liquid the length of
which is equal to the difference in length of the two tubes.

A somewhat different principle was adopted by E. Bouty in 1884. If a
current be passed through two resistances in series by means of an
applied electromotive force, the electric potential falls from one end
of the resistances to the other, and, if we apply Ohm's law to each
resistance in succession, we see that, since for each of them E = CR,
and C the current is the same through both, E the electromotive force or
fall of potential between the ends of each resistance must be
proportional to the resistance between them. Thus by measuring the
potential difference between the ends of the two resistances
successively, we may compare their resistances. If, on the oth