Carey Foster Bridge

It was invented by George Carey Foster

Q.What is resistance?

A.Resistance R=V \ I, where V is the potential difference applied across the wire and I is the current flowing through it due to the potential difference.

Q.What is specific resistance?

A.Specific resistance of a material is the resistance of one centimeter length of wire whose cross-sectional area is one square centimeter.

Q.What is the effect of temperature on resistance?

A.Resistance increases with increase in temperature of the material of wire.

Q.In what materials the resistance decreases with increase in temperature?

A.In Semiconductors

Q. If the radius of wire is doubled will the specific resistance change?

A.Specific resistance is independent of length or radius of wire it only depends on the material of the wire.

Q.Why is the resistance wire wire doubled before winding over the bobbin?

A.To avoid the inductive effects. The direction of current in one wire is opposite to the other hence the total flux is always zero.

Q.What is the principle of Carey Foster Bridge?

A.Position of null point changes on the wire when the resistance is connected in the outer gaps is interchanged.The difference in the value of resistance is equal to the resistances of the differences of bridge wire i.e (L-l).

Q.What is the principle of Wheatstone Bridge?

A.When the four resistances P,Q,R,S in the four arms of the Wheatstone bridge are such that there is no deflection in the galvanometer, we have:


Q.When is Carey Foster Bridge most sensitive?

A.It is most sensitive when P,Q,R,S are same or of the same order.

Q.Why the resistance for inner ratio arms be equal?

A.For increased sensitivity and accuracy.

Q.What is the minimum difference in resistances that can be measured by Carey Foster Bridge?

A.The resistance of one millimeter length of bridge wire

.Carey foster bridge


12 thoughts on “Carey Foster Bridge

  1. proved quite helpful apart for a question I didn’t find i.e. what is the null point of a galvanometer.
    maximum students say when no current flows through the galvanometer and galvanometer indicates zero, that particular point is Known as null point.
    But if we switch off the circuit and see on to the galvanometer then also we can see no current flowing through galvanometer and indicating zero.
    Students will have to be more precise on there answers.


    1. When the circuit is open , then obviously you won’t talk about any readings and all , + null point is actually a point , but when circuit is open , you won’t see any deflection throughout the wire , so ,
      that’s a difference, + Null point is actually when the potential difference between two points is 0 , so here we consider the point at which the jockey is touching on the bridge, (call it A) & the point where Galvanometer is initially connected (call it B).
      Also , Potential drop at every point on the bridge is different.


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