To determine the resistance of Weston type galvanometer by Kelvins method using Post Office Box Experiment Setup




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.

post office box-experiment

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,

post office experiment circuit

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:

postoffice experiment circuit diagram

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

observation table for post office xperiment

Result

The resistance of the Galvanometer is ............. Ω

Precautions

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