To determine the resistance of Weston type galvanometer by Kelvins method using Post Office Box.
Apparatus Required
1. Post Office Box
2. Galvanometer
3. Power Supply (2v)
4. Resistance Box
5. Connecting Wires
Theory
A post office box is a compact form of the Wheatstones bridge. In the instrument each of the arms AB & BC contains three of 10, 100 and 1000 Ω respectively as shown in Figure given below.
These are called ratio arms. The arm AD is the other ratio arm R. The key K1 is connected to the point A and the key K2 to the point B internally as shown by the lines drawn on the ebonite plate. The unknown, S, is connected between C & D, the battery, E, between C & A through the key K1 and the galvanometer, G, between D & B through the key K2. The circuit is now exactly the same as shown in figure below. Hence,
P / Q = R / G i.e G = R. (Q / P)
In the Kelvins method the same galvanometer whose has to be measured, is also used as an indicator for obtaining the balance point. The galvanometer acts as its own indicator for obtaining the balance point a shown in below image:
Procedure
1. Make connections as in the first figure of Post office box
2. Take resistance from box and keep other plugs tight.
3. Keeping R = 0, insert 10 Ωs in arms P & Q. Press tapping key K1 and adjust from resistance box so that deflection is between 10 & 30.
4. Insert 10 Ω in R, press tapping K1 and note deflection in galvanometer. If deflection decreases on tapping key K2, go on repeating the observation by increasing R in steps so that on pressing K1 first and then tapping K2, the deflection increases. The value of galvanometer lies between last two values of R.
5. Keeping Q = 10 Ω make P = 100 Ω. Starts with 10 times the lower value of R obtained in earlier step and increase it in steps, so that on pressing K1 first and then K2, the deflection increases instead of decreasing. Note last two values of R.
6. Keeping Q = 10 Ω make P = 1000 Ω. Starts with 10 times the lower value of R obtained in earlier step and increase it in steps, so that on pressing K1 first and then K2, the deflection increases instead of decreasing. Note last two values of R.
Observation Table
Result
The resistance of the Galvanometer is ............. Ω
Precautions
- The ends of the connecting wires should be clean.
- All the plugs should be properly tight.
- K1 should be pressed first and thereafter K2.
Post Office Box Experiment Viva Questions and Answers
Question 1: What is post office box?
Answer: It is a modified form of wheatstone bridge. It was first used in post office for determining the resistance of telephone wires and thus to find fault in them, so called post office box.
Question 2: Why it is called post office box?
Answer: Because it is first used in post office to find faults in telegraphic wires.
Question 3: What do you mean by resistance?
Answer: It is a property of a conductor virtue of which it offers obstruction to the flow of current.
Question 4: What is the difference between ohmic and non ohmic conductors?
Answer: The conductors which obeys ohms law are called conductors, whereas the conductors which do not obeys ohms law are called non-ohmic conductors. However, the relation V = IR is followed by both type of conductor.
Question 5: What is the balanced condition of bridge?
Answer: In balanced condition: P/Q = R/S
