A meter bridge is a pratical application of a Wheatstone bridge. It consists of a straight and uniform wire one meter long. The internal wire has a high resistivity and low temperature co-efficient of resistance (Manganin is the material used).
The wire is stretched along a meter scale with its ends soldered to two copper strips of neglible resistance between the copper strips and third one fixed along the panel have two gaps AB and CD.
The unknown resistance is connected in the gap AB and a suitable resistance is connected in the gap. There is a jockey which series to make contact will the stretched wire and a pointer attached it to indicates we position of the point of contact with scale.
The position of jockey is adjusted until the galvanometer shows no deflection. Since the wire is of uniform thickness, the resistance is proportional to length.
The determination here may be inaccurate due to the following errors:
(a) Non uniformaty of the wire. To eliminate this error the meter bridge wire must be calibrated in an auxillary experiment.
(b) The position of the point where the jockey makes contact with the wire may not be shown accurately by the pointer attached to the scale.
This error may be eliminated by interchanging R and S and repeating the experiment. The mean of two gives the accurate balancing length.
(c) The ends of the wire may not coincide will the ends of the scale and there may be a small resistance where the wire is connected to the copper strip. This resistance is known as END correction. To determine we take R and S known and obtain a balance point a length L1 of the melor bridge wire. If α and β are end corrections at the two ends we have,
Now interchanging the positions of R and S we obtain a balance at a length L2 of the meter bridge wire.
Solving these two equations for α and β we get
(d) If a current is passed for a longer direction of time the point may flicker. To avoid this the current in the wire may be kept as small as possible and the key should be pressed for as short a time as possible while taking a reading.
