(i) The meter bridge is another form of Wheatstone's bridge. It consists of a uniform manganin wire AB of one meter length.
(ii) This wire is stretcheed along a meter scale on a wooden board between two copper strips C and D. Between these two copper strips another copper strip E is mounted to enclose two gaps `G_(1) and G_(2)` as shown in Figure.
(iii) An unknown resistance P is connected in `G_(1)` and a standard resistance Q is connected in `G_(2)` . A jocky (conducting wire) is connected to the terminal E on the central copper strip through a galvanometer (G) and a high resistance (HR).
(iv) The exct positive of jockey on the wire can be read on the scale. A Lechlanche cell and a key (K) are connected across the ends of the bridge wire.
(v) The position of the jocky on the wire is adjusted so that the galvanometer shows zero deflection. Let the point be J. The lengths AJ and JB of the bridge wire now replace the resistance R and S of the Wheatstone's bridge. Then
`(P)/(Q)=(R)/(S)=(R'.AJ)/(R'.JB) " "...(1)`
where R' is the resistance per unit length of wire
`(P)/(Q)=(AJ)/(JB)=(l_(1))/(l_(2))`
`P=Q(l_(2))/(l_(2)) " "....(3)`
(vi) The bridge wire is soldered at the ens of the copper, strips. Due to imperfect contact, some resistance might be introduced at the contact. These are called end resistances. This error can be eliminated, if another set or readings are takne with P and Q interchanged and the average value of P is found.
(vii) To find specific resistance of the material of the wire in the coil, P the radius r and length 1 of wire is measured. The specific resistance or resistivity `rho` can be calculated unsing the relation
Resistance `=rho(l)/(A)`
By rerrangiing the above equation, we get `rho=` Resistance `xx (A)/(l) " "...(4)`
If P is the unknown resistance equation (4) becomes,
`rho=P(pi r^(2))/(l)`
