Experiment E17 (E1): Measuring Unknown Resistances by Post Office Box (P.O. Box)
Q1: What is a P.O. Box and why is it so called? Ans: It is a compact form of the Wheatstone bridge. It was originally intended to measure the resistance of telegraphic wires in the British post office.
Q2: On what principle it works? Ans: It works on Wheatstone’s bridge principle.
Q3: Why the battery key pressed first and the galvanometer key afterwards? Ans: The battery circuit should be completed before the galvanometer circuit to avoid the effect of self-induction.
Q4: What condition must be satisfied to get a balance point? Ans: The condition must be satisfied to get a balance point according to the principal of Wheatstone’s bridge. The unknown resistance is given by , where and are the fixed known resistance in the ratio arms and is a variable resistance of known value.
Q5: Is it suitable for measuring high or low resistance? Ans: Neither very high nor very low resistance can be measured, because the galvanometer will show deflection within a certain range of resistance.
Q6: Will your null point change when a cell whose E.M.F. is gradually decreasing or increasing? Ans: No, because the distribution of potential at the two parallel branches will hang equally, maintaining the null point constant.
Q7: If the resistance of the P.O. Box were calibrated at will they give the same value at other temperature? Ans: No, the resistance of materials increase with the increase of temperature. The resistance is expressed as .
Q8: On what condition Wheatstone bridge is more sensitive? Ans: The sensitiveness of the bridge is affected by the resistance of the galvanometer and battery. The lower the resistance, the greater the sensitivity of the bridge.
Q9: What are the advantages of the Wheatstone bridge method of measuring resistance over other methods? Ans: In the Wheatstone bridge method, a cell of any kind may be employed. The zero potential difference at the ends of the galvanometer will remain unaffected and there will be no change in the null point.
Experiment E18 (E2): Comparing E.M.F.’s of Two Cells by Potentiometer
Q1: What do you mean by the term E.M.F. of a cell? Ans: E.M.F. of a cell is defined to be the potential difference in open circuit.
Q2: What is the difference between potential difference and E.M.F.? Ans: E.M.F. is present even in the open circuit, but potential difference is present only in the closed circuit. E.M.F. is the cause, but potential difference is the effect.
Q3: What is the function of high resistance in series with the galvanometer? Does the null point depend on its value? Ans: The high resistance does not in any way interfere the position of the null point. It protects the galvanometer from damage.
Q4: What is the driver cell? What is its function? Ans: The cell is the driver cell. Its function is to send current in the circuit so that there is a drop of potential between potentiometer terminal A and B.
Q5: Knowing the E.M.F. of one cell can you find the potential drop () per unit length of the wire? Ans: Yes, from the relation or . Knowing and can be found.
Q6: How can you increase the sensitiveness of the potentiometer? Ans: By increasing the balancing length of the potentiometer as long as possible. The sensitiveness of the potentiometer increases.
Q7: What kind of wire should be used as the potentiometer wire and why? Ans: Eureka, or Manganin etc., because they have high resistance. Otherwise, the fall of potential will not be appreciable even for the whole length of the potentiometer wire.
Experiment E19 (E3): Resistance per Unit Length by Carey Foster’s Bridge
Q1: What is Carey-Foster’s bridge? Ans: It is a modification of the meter bridge designed to measure the difference between two resistances of nearly equal values.
Q2: What should be the order of the resistance P and Q? Ans: Two resistance boxes and nearly equal resistance of 5 or 10 ohms are connected in the inner gaps.
Q3: What is ? Ans: is the resistance per unit length of the wire.
Q4: Will the presence of the end error in the bridge influence your result in any way? Ans: No, it is evident from the theory that it is independent of end error.
Q5: Why is it not desirable to measure with P.O. Box? Ans: P.O. Box method is only a rough method and not accurate.
Q6: At what temperature you have done your experiment? Ans: At room temperature.
Q7: Can you compare two nearly equal resistances by this method? If so what is the limitation? Ans: Yes, but their values should not differ by more than the total resistance of the bridge wire.
Q8: When the bridge is most sensitive? Ans: The bridge is most sensitive when the four arms are of equal resistance, the battery and the galvanometer resistance each equal to that of one of the arms.