Aim:-
A post office box and other necessary instruments are given to you. Take three appropriate ratios of the resistances in the two arms of Post Office Box and determine the values of two unknown resistances.
Now connect the unknown resistances in series and parallel respectively and verify the laws of series and parallel combinations of resistances.
PRINCIPLE :-
It works on the principle of balanced Wheatstone bridge.
APPARATUS :-
A Battery,
unknown resistance (Rx & Ry),
Post Office Box,
a galvanometer,
rheostat,
connecting wires.
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| Post Office Box Apparatus |
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| Condition 1 Resistance Rx connect between C and D |
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| Post Box Wiring Diagram find Unknown Resistance Rx |
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| Condition 2 Resistance Ry connect between C and D |
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| Post Box Wiring Diagram find Unknown Resistance Ry |
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| Post OfficeBox line diagram Series connection Rx and Ry |
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| Series connection of Rx and Ry wiring diagram |
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| Parallel Connection line Diagram of Rx and Ry |
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| Wiring Diagram Parallel connection of Rx and Ry |
CALCULATIONS :-
Series connection, R’s =
Rx + Ry + …..Ohm
Parallel connection, R’p
= Rx Ry / Rx + Ry = ….Ohm.
RESULT :-
(1) Unknown resistance,
Rx = ….Ohm. , Ry = ….Ohm
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| Observation for Post Office Box Experiment |
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| Calculation table for Post Office Box |
CONCLUSION :
The experimental and theoretical values of equivalent resistance for series and parallel connections are equal,within the experimental error. This implies the validity of the rules for the different combinations of resistances.
Precautions
All plug keys of Post Office Box must be tight.
Contact points of K1 and K2 must be cleaned with sand paper.
Galvanometer should be very sensitive.
Throughout the experiment press key K2(battery key) first and then key K1(galvanometer key).
More Video : Post Office Box Video presentation.
Post Office Box Experiment.
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