In this topic, you study the definition, theory, types & diagram of the post office box and how it is used to find out the value of a unknown resistance.
The post office box is the another example of wheatstone bridge principle. In this instrument all the three unknown resistances P, Q and R are suitably arranged in a resistance box of special design as shown in Fig. 23.24. The unknown resistance forms the fourth arm of the bridge. In the top row of the instrument, there are six resistances in two sets of three resistances of 10, 100 and 1000 ohms. These two sets are arranged symmetrically round the centre of row as shown. These sets are called the ratio arms, It represents P and Q of the wheatstone bridge. The remaining resistance of 1 to 5000 Q are arranged as an ordinary resistance box. These resistances corresponds to the third unknown resistance R, The unknown resistance X is connected between the A and C. The galvanometer is connected between point B and D. The battery is connected between A and C. nere are two keys provided for galvanometer and cell. known as Galvanometer key and cell key respectively.
Working. To make the determination of the unknown resistance, make the connection diagrams as shown in Fig. 23.24. Take out two plugs from
Fig. 20.34. Post office box.
the resistance arm, 10 and 10. Press cell key and then galvanometer key, see the deflection, now remove the infinity plug and press the keys and note the deflection. It should now be in reverse direction. It indicate the correctness of the connections. The resistance in the portion R Of the post office is adjusted till the null point in the galvanometer. If no null point is achieved then, find out two resistances differing by one ohm which will give opposite deflection in the galvanometer. Now change the ratio from 10 : 10 to 10 : 100, and the reading will be now between the multiple of 10. First say the deflection was changing its direction between 1 and 2. Now find the position between 10 onward, let it be coming at 15 and 16. Again change the ratio arm from 10 : 1000 the reading can be obtained between HO and 160. Find the null point let it is 153 and 154. Now the average of the readings is 153.5 and the correct value is
Thus we can find out the unknown resistance.
