**Viva Questions and Answers for Meter Bridge Experiment**

**(i) Why is Wheatstone Bridge so called?**

Ans: The principle was discovered by Sir Charles Wheatstone

**(ii) What is a Metre Bridge. Why is it so called?**

Ans: It is an instrument for comparing resistances. The bridge wire has a length of one metre

**(iii) What is the principle of Wheatstone Bridge?**

Ans: When the bridge is balanced P/Q = R/S

**(iv) Can you find very high resistances accurately with the help of a metre bridge?**

Ans: No

**(v) Why is the galvanometer is graduated both sides of zero?**

Ans: If the jockey is pressed on either side of balance point the deflection in the galvanometer are in opposite directions

**(vi) Can we use an ammeter instead of a galvanometer?**

Ans: Ammeter is not sensitive compared with a galvanometer. Moreover it is graduated with zero at one end

**(vii) What is the material of the bridge wire: What is the criterion for selections?**

Ans: Manganin wire is used. Low temperature coefficient

**(viii) Why are copper strips on the bridge thick?**

Ans: To minimise resistance

**(ix) Why is it desirable to have null point as near the centre as possible?**

Ans: This will minimise end corrections. Interchanging the resistance also minimises end correction.

**(x) When is Wheatstone Bridge most sensitive?**

Ans: It is most sensitive when P, Q, R and S are of the same order of magnitude

**(xi) What are the laws of combination of resistances?**

Ans: R = R_{1} + R_{2 }for series connection and 1/R = 1/R_{1 }+ 1/R_{2} for parallel connection.

**(xii). What is the principle of a metre bridge?**

Ans: Wheatstone's principle.

**(xiii). What is the material of the wire of metre bridge? Can we use copper wire?**

Ans: The wire of metre bridge is generally made of an alloy constantan or eureka. The resistivity of copper is very low and hence it cannot be used in metre bridge.

**(xiv). Why do we prefer the null point in the metre bridge experiment in the middle of bridge wire?**

Ans: When the null point is in the middle, the ratio arms are nearly equal and the end resistances produce negligible effect.

**(xv). Why is it essential that bridge wire should be of uniform area of cross section?**

Ans: In deriving the formula, we assume that resistance is proportional to length of the wire. This is true only when the wire is of uniform cross sectional area throughout.

**(xvi). Why is metre bridge so called?**

Ans: Because bridge uses one metre long wire.

**(xvii). When is a Wheatstone's bridge said to be balanced?**

Ans: A Wheatstone's bridge is said to be balanced, when the ratio of resistances in arms balance or become equal.

**(xviii). What are the applied forms of a Wheatstone's bridge?**

Ans: i. Post Office Box

ii. Metre Bridge

**(xix). Define electrical conductivity.**

Ans: The reciprocal of resistivity is called electrical conductivity.

**(xx). What is null point?**

Ans: Null point is a point on the wire, keeping jockey at which the galvanometer gives no deflection.

**(xxi). How are three resistances connected to get a maximum effective resistance?**

Ans: They are connected in series.

**(xxii). How does resistance change in series combination?**

Ans: Resistance increases.

**(xxiii). How does resistance change in parallel combination?**

Ans: Resistance decreases.

**(xxiv). Explain increase of resistance in series combination.**

Ans: In series combination, the effective length of the resistor increases. So resistance increases.

**(xxv). Explain decrease of resistance in parallel combination.**

Ans: In parallel combination, the effective area of cross section of the resistor increases. So resistance decreases.

**(xxvi). What is the effective resistance of two equal resistance connected in parallel?**

Ans: Half of a resistance

**(xxvii). What is the effective resistance of two equal resistance connected in series?**

Ans: Twice of a resistance

**(xxviii) Aim of Meter Bridge Experiment**

Ans:

(a)To determine the resistivity of the material of a wire

(b) To verify the laws of resistances.

**(xxix) Apparatus of Meter Bridge Experiment**

Ans: The metre bridge, resistance box, resistance wire, battery, high resistance (HR), key etc.

A metre bridge contains a uniform resistance wire, AB having 1 m length, kept extended between two copper strips set on a wooden board. Another copper strip set centrally on the board leaves two gaps amid the strips. The copper strips are provided with terminals.

**(xxx) Theory of Meter Bridge Experiment to determine the resistivity**

Ans: If l is the balancing length of the bridge wire from the side of the unknown resistance X and R is the known resistance,

X/R = l/(100 —l);

Therefore, X = R [l/(100 —l)]

Resistivity of X is ρ = X x (πr^{2} / l); where r is the radius and l is the length of the wire X.

**(xxxi) Procedure of Meter Bridge Experiment to determine the resistivity of the material of a wire.**

Ans: The unknown resistance X is connected in the left gap and a resistance box R in the right gap. A battery is connected between A and B. A galvanometer is connected between C and a jockey that can slide along the bridge wire.

A suitable resistance R is taken in the box and circuit is closed. The position of the jockey is adjusted so that the galvanometer shows null deflection.

If l1 is the length of balance measured from the left end A, then,

X /R = l_{1}/(100 — l_{1})

R and X are interchanged in gaps and the balancing length l2 from the right end B is measured. Then,

X/R = l_{2}/(100 — l_{2})

If l = (l_{1} +l_{2})/2;

then X/R = l/(100 — l)

Therefore, X— R[l/(100 — l)]

The experiment is repeated for different values of R; but the balancing point must be near the middle of the bridge wire. In each case X is calculated. From these, the mean value of X is calculated. The radius r of the wire is measured using screw gauge. The length `L' of the wire is measured by a metre scale. Resistivity of the wire is calculated using the equation,

ρ = X πr^{2}/L