The practical form of Wheatstone’s bridge is slide wire bridge.
Slide wire bridge: It consists of uniform wire AC, usually one metre long, soldered to the ends of two thick rectangular strips of copper A and C, fixed on a wooden base.
A sliding contact B, called the jockey, can be moved along a graduated scale. By pressing a small key mounted on the jockey, contact can be made at any point along the length of the wire.
If the length of the wire is l metre, it is called meter bridge. The unknown resistance is placed across the gap DC and a known resistor R is placed across the gap AD. The two resistances P and Q are obtained from the wire AC by moving the jockey B connceted to the strip D through a galvanometer G.
Working: The key K in the cell circuit is inserted and the jockey is moved along the wire till, for a certain position B, that galvanometer shows no deflection. The bridge is then said to be balanced, if P and Q are the resistances of the parts AB and BC of the wire, we have
\(\frac{P}{Q} = \frac{R}{S}\)
If the slide wire is of uniform cross-section, the resistance, of AB and BC shall be proportional to their lengths, say l and (100 - l) respectively. If r is the resistance per cm length of the wire, we have
\(\frac{P}{Q} = \frac{lr}{(100-l)r}= \frac{l}{100-l}\)
Substituting in the above equation, we have
\(\frac{l}{100-l} = \frac{R}{S}\)
or S = \(\frac{100-l}{l}R\)
Knowing l and R; S can be calculated.
This arrangement cannot be used for determining very low resistance because this apparatus is most sensitive only when all the resistances of all arms [P, Q, R and S] are comparable i.e. neither too large, nor too small.
Advantage: It is a null method and therefore it is accurate.
