Page:Outlines of Physical Chemistry - 1899.djvu/213

 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'.

�� �