o a minimum by winding the
wire of the coil backwards and forwards in alternate layers.

[Illustration: FIG. 1.]

With these arrangements the usual method of measuring resistance by
means of Wheatstone's bridge may be adapted to the case of electrolytes.
With alternating currents, however, it is impossible to use a
galvanometer in the usual way. The galvanometer was therefore replaced
by Kohlrausch by a telephone, which gives a sound when an alternating
current passes through it. The most common plan of the apparatus is
shown diagrammatically in fig. 1. The electrolytic cell and a resistance
box form two arms of the bridge, and the sliding contact is moved along
the metre wire which forms the other two arms till no sound is heard in
the telephone. The resistance of the electrolyte is to that of the box
as that of the right-hand end of the wire is to that of the left-hand
end. A more accurate method of using alternating currents, and one more
pleasant to use, gets rid of the telephone (_Phil. Trans._, 1900, 194,
p. 321). The current from one or two voltaic cells is led to an ebonite
drum turned by a motor or a hand-wheel and cord. On the drum are fixed
brass strips with wire brushes touching them in such a manner that the
current from the brushes is reversed several times in each revolution of
the drum. The wires from the brushes are connected with the Wheatstone's
bridge. A moving coil galvanometer is used as indicator, its connexions
being reversed in time with those of the battery by a slightly narrower
set of brass strips fixed on the other end of the ebonite commutator.
Thus any residual current through the galvanometer is direct and not
alternating. The high moment of inertia of the coil makes the period of
swing slow compared with the period of alternation of the current, and
the slight periodic disturbances are thus prevented from affecting the
galvanometer. When the measured resistance is not altered by increasing
the speed of the commutator or changing the ratio of the arms of the
bridge, the disturbing effects may be considered to be eliminated.

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

The form of vessel chosen to contain the electrolyte depends on the
order of resistance to be measured. For dilute solutions the shape of
cell shown in fig. 2 will be found convenient, while for more
concentrated solutions, that indicated in fig. 3 is suitable. The
absolute resistances of certain solutions have be