Specific resistance of the material of a wire using a meter bridge

A known length (L) of a wire is connected in one of the gaps (P) of a metre bridge, while a ‘resistance box’ is inserted into the other gap (Q). The circuit is completed by using a battery (B), a rheostat (Rh), a key (K) and a galvanometer (G).

Alt txt:  specific-resistance-of-material

The balancing length () is found by closing key K and momentarily connecting the galvanometer until it gives zero deflection (null point). Then,  p / q =  / 100 –  (using the expression for the meter bridge at balance)

[Here, P represents the resistance of the wire while Q represents the resistance in the resistance box. The key K is kept open when the circuit is not in use.]

The resistance of the wire, P = r L / πr² or r = πr² / L P

where r is the radius of the wire and L is length of the wire (r is measured using a screw gauge while L is measured with a scale).