WHEATSTONE BKIDGE
��Then, according to Ohm's law, in the conductors acb and a d b we have :
��Therefore,
��c
�=
�P— P
r
�_p— P'
- r' '
�o'
�-s
�p— p
R
�p— p'
"" R' -
� � �r _
R
�r'
= R'"
��Since the points c and d are at the same potential, there will be no current in the bridge, and the galvano- meter will show no deflection. Conversely, when in such
an arrangement the galvanometer is at zero, the points cand d are at the same potential, and the ratio - =-
R R
exists between r, r', r and r ; .
In practice, r and r' are known resistances of 1, 10, 100,
1 ,000 ohms which can be altered as desired. For r
a resistance-box is used, and r' is the resistance to be measured. The keys in the resistance-box are then arranged so that the galvanometer indicates no current in the bridge, and r, r', and r being known we can calcu- late r'.
�� �
