N.B. — If an ordinary metre bridge is used, r,, r.^ will form the
stretched wire, but it is not suitable to use with the balancing cell.
In such cases one similar to Fig. 169 should be used. If possible
the source of alternating current B should be removed to a dis-
tance from the operator, so that the noise of the " contact breaker"
may not interfere with the hearing at the telephone. It will, for



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Practical Electrical Testitu



129



this reason and that of obtaining greater sensitiveness, be an
additional help if two telephones are used in parallel, one to each
.ear. Vide precautions (p. 130) to be adopted.
Observations. — (i) Connect up as in Fig. 61, or, if the
special bridge (p. 345) is used, then connect as there described.
If the present arrangement is used,
make r^ = r.;,, and adjust R so that
no sound or niiniiiiiim sound is heard
in Gt, and note the values of R and
temperature and of rj, ;■„ for reference
only.
N.B.— The test will be most
sensitive when r-^ and i:, are only a
little greater than the value of R
required to balance, for then the arms
of the bridge will be more nearly
equal in resistance. Balance should then be disturbed by altering
R, and re-obtained thus two or three times, and the mean value
of R noted, which will thus be more accurately equal to the
difference in the resistance of C and c.
(2) Repeat (i) for four or five different temperatures of the
bath or solution.
