In use the resistances r, r', and s are made to vary as desired. To
measure an unknown resistance it is introduced at S', and one of the
other resistances is varied until the galvanometer is unaffected. Then
the resistance of S' is determined by calculation as just explained. The
artificial resistances may be resistance coils, q. v., or it is enough
to have one unknown resistance at s. Then if the length of wire ABC is
accurately known, the point B can be shifted along it until the balance
is attained. The relative lengths A B, and B C, will then give the ratio
r : r' needed for the calculation. This assumes the wire ABC to be of
absolutely uniform resistance. This is the principle of the meter-bridge
described below. The use of coils is the more common method and is
carried out by special resistance boxes, with the connections arranged
to carry out the exact principle as explained. The principle of
construction and use of a resistance box of the Wheatstone bridge type,
as shown in the cut, is described under Box Bridge, q. v.