Carey Fosters Bridge Experiment Questions and Answers for Viva

Answer-1: A bridge based on the principle of Wheat stones bridge that is used to compare two nearly equal resistances and to determine values of low resistances and the specific resistance of a wire. It differs from a meter bridge because additional resistances of similar magnitudes are included at either end of the meter wire.

Answer-2: There is no great difference between the two in principle but,
1. A Carey fosters bridge there is no end resistance whereas in the case of a meter bridge end resistance affects our readings. Hence Carey foster bridge is more accurate than meter bridge.
2. Carey foster bridge can also measure low resistance and high resistance but a meter bridge cannot do so.

Answer-3: A Carey-fosters bridge is a modified version of the meter bridge (Wheatstone bridge or potentiometer).
1. It can be used to find out the average resistance per unit length of the meter bridge wire and hence find out the unknown resistance.
2. It is more accurate and more sensitive than a meter bridge.

Answer-4: The Carey Foster bridge works on the principle of the Wheatstone bridge (potentiometer)which can be used to measure fractional resistance.

Answer-5: Null method means zero current through the galvanometer. So the calibration of the galvanometer does not come into play.

Answer-6: To eliminate the effect of current due to thermo e.m.f.

Answer-7: This is due to the finite resistance of the copper strips fixed within the Carey Fosters bridge, the resistance at the junctions of the wires with the copper strips and the non coincidence of the ends of the wire with the zero and hundred division marks on the scale.

Answer-8: No, because thermoelectric voltages developed at the junctions of dissimilar metals may cause problems when low resistances are measured. The resistances of leads and contacts external to the bridge circuit may also affect measurements of very low resistances.

Answer-9: Very high resistances cannot be measured very accurately with a standard Wheatstone bridge due to leakage of currents. That is current leakage in the electrical insulation may be comparable to the current in the branches of the bridge circuit when high resistances are measured. The sensitivity of the bridge to balance is also reduced for high resistances.

Answer-10: No, since copper has a low specific resistance and a high temperature coefficient.

Answer-11: When the resistance in the outer gaps is interchanged, the position of the null points on the bridge wire shifts. The difference between the two resistances is equal to the resistance of the bridge wire connected between these two null points.

Answer-12: By doing this accuracy of the result will be increased.

Answer-13: The sensitivity and accuracy of a meter bridge is directly proportional to the length of the bridge wire. In Carey Foster Bridge, the effective length of the bridge wire is increased and so it is more accurate and sensitive.

Answer-14: So that it may offer negligible resistance.

Answer-15: Carey-Foster Bridge is more sensitive than the wheat stone bridge because the two resistances are also connected in the end gap in series with bridge wire which increases the bridge wire virtually which makes it more accurate too.