PONT PHYSIQUE 

bridge circuit is a type of electrical circuit in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them

Wheatstone Bridge
The Wheatstone bridge is an electrical circuit for the precise comparison of resistances.It was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone

CONSTRUCTION:

The Wheatstone bridge is an electrical bridge circuit used to measure resistance. It consists of

1. a common source of electrical current (such as a battery) 
2. a galvanometer that connects two parallel branches, containing four resistors,of which two are known R_1and R_3.One of the parallel branches contains one adjustable resistor R_2 and an unknown R_X.

 WORKING:

BALANCING THE BRIDGE:

If the bridge is unbalanced, the direction of the current indicates whether R_2 is too high or too low. R_2 is varied until there is no current through the galvanometer, which then reads zero.

If the ratio of the two resistances in the known leg (R_2 / R_1) is equal to the ratio of the two in the unknown leg (R_x / R_3), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer V_g.Such a bridge is called BALANCED.

Detecting zero current with a galvanometer can be done to extremely high accuracy. Therefore, if R_1, R_2 and R_3 are known to high precision, then R_x can be measured to high precision. Very small changes in R_x disrupt the balance and are readily detected.

At the point of balance, the ratio of R1 to R2 is equal to R3 and R4.

METER BRIDGE
It consists of a meter long wire of high resistance and low temperature co-efficient.

CONSTRUCTION:

• It consists of a wire AB of 1 meter length and uniform cross section.
• A battery of emf ' e ' , a plug key 'K' are connected between the two terminals A and B.
• A graduated meter scale S is fixed by the side of the wire for taking the lengths of the wire from the +ve terminal i.e A. 
• Three strips C1 ,C2 ,C3 of copper or bronze with negligible resistances are also stretched on the board with gaps in between them. 

• A resistance box 'RB' is connected in the gap 'G1' . Resistance in RB = P 
• The unknown resistance 'X' is connected in gap 'G2' .
• In between the centre C of strip C2 , a galvanometer G and a high resistance 'H.R' are connected in series . 

The other end of the galvanometer is connected to a ‘jockey’ which is essentially a metallic rod whose one end has a knife-edge which can slide over the wire to make electrical connection.

• When the jockey is at a point 'D' on the wire , it divides the wire into two parts AD and DB of lengths L1 and L2.
• Resistance of AD length of wire =R = L1 [sigma] 
• Resistance of DB length of wire = S = L2 [sigma]  

where [sigma] is the resistance per unit length of the wire.

Now , the circuit is exactly similar to a Wheatstone bridge.

WORKING:

The jockey is now pressed 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 . 

CALCULATIONS:

The length l1 from A to D is noted . The unknown resistance X can be calculated from the following equation.
When the bridge is balanced , we have

     [(P)/(X)]   =  [(R)/(S)]   =  [(l1)/(l2)]

But in a meter bridge l2 = ( 100 - l1 ) . So the balanced condition of a meter bridge is

[(P)/(X)]   =  [(l1)/(100-l1)]

USES:

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 .