Meter bridge

class 12 th physics meter bridge

1. Priyanka Jakhar Physics Lecturer GGIC Vijay Nagar Ghaziabad U.P. Meter Bridge Class 12th

2. Meter Bridge It is also known as  slide wire bridge. It is an instrument which works on the principle of a Wheatstone bridge . A meter bridge is used in finding the unknown resistance of a conductor as that of in a Wheatstone bridge. Principle:-- works on the principle of a Wheatstone bridge . One Lateral pair of resistance is replaced with the straight wire of a specific length of 1m uniformly across the section. One pair contain a known resistances and other pair contain Unknown resistance.

3. Construction of meter Bridge Wire :-- Meter bridge consist of a one meter long manganin  or eureka straight wire with a uniform cross sectional area ,which is fixed on the wooden block. wooden block :-- All the connections made on it. Metal strips :-- Two L shaped metal strips one and straight metal strip to make the Wheatstone bridge. Galvanometer :-- The galvanometer is the device used for detecting the presence of small current and voltage or for measuring their magnitude. The galvanometer is mainly used in the bridges and  potentiometer  where they indicate the null deflection or zero current. jockey stick :-- The jockey is used to slide on the bridge wire. It is a metal rod with one end as knife edge. Connecting wires :-- Use to complete the circuit to flow of current. Resistance box :-- resistance box is that the variable resistances are available at one point. If any circuit requires variable resistances, then there is no need of replacing the resistor. Unknown resistance wire :-- A wire used to find the resistance.

4. The Meter bridge consists of a wire of length 100 cm and of uniform cross-sectional area clamped between two thick metallic strips A and C. ● The metallic strip has two gaps across which two resistors R and X are connected. ● Here X is the unknown resistance whose value to be determined. ● X is the standard known resistance. ● A galvanometer is connected between the central metallic strip to the sliding jockey J, which slides over Meter bridge wire. ● The four arms AB, BC, DA and CD (with resistances R, S, resistances of wires of length l and ( 100 - l ) form a Wheatstone bridge. ● If jockey is moving along the wire, when bridge is balanced. ● By measuring l, the unknown resistance R can be measured. By Wheatstone s bridge P R = Q S Where P = R Q = X R = ρ l / A S = ρ ( 100 - l ) / A R R AJ X R JB

5. R l = X ( 100 – l ) l R = X ( ) ( 100 – l ) A B R.B (R) X K E l cm 100 - l cm G J ( 100 – l ) X = R ( ) l R ρ l / A = X ρ ( 100 - l ) / A

6. A B R.B (R) X 2 G J K E l cm 100 - l cm X 1 R ρ l / A = X ρ ( 100 - l ) / A R l = X ( 100 – l ) l R = X ( ) ( 100 – l ) ( 100 – l ) X = R ( ) l X = X 1 + X 2 When two unknown resistance wire are in series

7. A B R.B (R) X 2 G J K E l cm 100 - l cm X 1 X 1 X 2 X = + X 2 X 1 R ρ l / A = X ρ ( 100 - l ) / A R l = X ( 100 - l ) l R = X ( ) ( 100 – l ) ( 100 – l ) X = R ( ) l When two unknown resistance wire are in parallel

8. The material of wire used in meter bridge is made of  manganin  or eureka due to there high specific resistance or resistivity. Thick copper strips are used in meter bridge because copper is a good conductor of electricity. Application of a meter bridge • It is used to measure an unknown resistance • It can be used for the or comparison of  two unknown resistances . Note :-- Clean the connecting wires and the connecting points of meter bridge properly with sand paper. All connections should be neat and tight. Balance point should lie between 30 cm and 70 cm. Do not keep the jockey and the wire in contact for a long time. Hold the jockey perpendicular to the wire of meter bridge.