Saturday, July 16, 2011

Meter Bridge

Meter Bridge
Introduction to Meter Bridge
The working of a Meter Bridge is based on the principle of Wheat stone bridge . It is a simpler form of Wheat stone bridge and is more accurate because , the resistances R and S can be changed in a continuous fashion . It is called a meter bridge it consists of a wire AB of one meter length .
The meter bridge as shown in figure , consists of three thick strips C 1 , C 2 and C 3 made of copper or brass . The resistances of these strips are almost zero . Theshapes and positions of the strips fixed on a wooden board are as shown in figure . A manganin wire of uniform cross section and 1 m length is stretched tight between the two terminals A and B . A jockey J slides over the wire . A graduated meter scale S is fixed by theside of the wire for taking the lengths of the wire . Thelengths are to be measured from th end where +ve terminal of battery is connected i.e. end A in the figure . A battery of emf ' ' , a plug key 'K' are connected between the two terminals A and B . A resistance bax 'RB' is connected in the gap 'G 1 ' . The unknown resistance 'X' is connected in gap 'G 2 ' . In between the centre C of strip C 2 and the jocky 'J' , a galvanometer G and a high resistance 'H.R' are connected in series . When the jockey is at a point 'D' on the wire , it divides the wire into two parts AD and DB of lengths l 1 and l 2 .
As the wire is of uniform cross section , we can haveresistance of AD length of wire =R = l 1 and resistance of DB length of wire = S = l 2 where is the resistance per unit length of the wire . Now , the circuit is exactly similar to a Wheatstone bridge withresistance in RB = P .
Unknown resistance X = Q and R = l 1 and S = l 2 .
When the bridge is balanced, we have
= =
But in a meter bridge l 2 = ( 100 - l 1 ) . So the balanced condition of a meter bridge is =
Experimental Procedure for Meter Bridge
A suitable value of resistance P is kept in the resistance box . In the beginning , the high resistance H.R is included in series with the galvanometer in the circuit . The connections are checked for deflections in galvanometer by pressing the jockey first near A and then at B . If these deflections are in opposite directions , the connections are proper . Otherwise , theconnections are to be checked . The jockey is nowpressed at various points one the wire from 'A' to towards 'B' , until we get near null deflection in the galvanometer . At this stage, the high resistance is shunted and the exact balance point giving null deflection at D is obtained . The length l 1 from A to D is noted . The unknown resistance X can be calculated from equation (1) .
Specific resistance of the material of a given wire can also be determined using meter bridge . We keep the given wire in gap G 2 and determine its resistance (R) as described above . The specific resistance of the material of the wire ( ) is calculated from
The following precautions are to be observed with a meter bridge experiment .
*. The current must be kept at low value . Otherwise , resistance of wire changeson getting heated and affects the values .
*. The current should not be passed continuously for a very long time . The wire gets heated and values will be altered .
*. The jockey should not be dragged along the wire . Otherwise , the wire gets deformed and its resistance gets altered .
Importance of High Resistance (hr) in Meter Bridge
The galvanometer is a verysensitive current meter . It should be protected from high currents flowing through it . For this reason ,a high resistance is includedin series with the galvanometer . When the balancing point is not obtained , because of H.R in series , the current through galvanometer will be small . But , when the near balance is obtained , the H.R is to be shunted for getting an accurate value for balancing length . Now , there will be no damage to galvanometer , because already near balance is achieved and current through galvanometer will be very small .
The meter bridge can be conveniently used to
*. determine an unknown resistance
*. compare two resistance
*. determine the specific resistance of the material of a wire .
Related Questions
How to measure unknown resistance in meter bridge?
The wheatstone bridge or metre bridge
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  1. Why is the wire 1 meter? Is it a compulsion or other length can be considered as well?

  2. This comment has been removed by the author.

  3. what is the applications of the metric bridge?

  4. How can we find the null value at 5 ohms, from A-B from where the resistance wire is placed on the ruller