Law of Combinationof Resistors. (Wheatstone's Bridge),Video for std 8 to 12


Practical - 3
LAWS OF COMBINATION OF RESISTORS
( WHEATSTONE’S BRIDGE & POST OFFICE BOX )

AIM :-
You are given a Wheatstone Bridge and other necessary instruments. Find the values of the unknown resistance Rx and Ry . Now connect them in series and parallel respectively and measure their resultant resistances and test the laws of series and parallel connections.
PRINCIPLE :-     It works on the  principle of a balanced Wheatstone bridge. 

Apparatus :-   Wheatstone-bridge (meter-bridge),
                         2 Volt Battery,
                         Rheostat,
                        Galvanometer,
                        Jockey,
                        Key,
                        Resistance box(1 – 100 W),
                        Resistance coil.


Wheatstone Bridge Apparatus



(1)    Condition :- Connect unknown resistance Rx between  M – N.
Wheatstone Bridge line Diagram (Condition-1)
Wheatstone Bridge Wiring Diagram (Condition-1)


(2)    Condition :- Connect unknown resistance Ry between  M – N.
Wheatstone Bridge line Diagram (Condition-2)
Wheatstone Bridge Wiring Diagram (Condition-2)


(3)    Condition :- Connect unknown resistance in Series
Wheatstone Bridge line Diagram (Series connection)

Wheatstone Bridge Wiring  Diagram (Series connection)


(4)    Condition :- Connect unknown resistance in Parallel .
Wheatstone Bridge line Diagram (Parallel connection)


Wheatstone Bridge Wiring Diagram (Parallel connection)

                                                                                                                            

OBSERVATION TABLE
Wheatstone Bridge Observation table

CALCULATIONS :-
(1       (1) Series connection, R’s = Rx + Ry + …..W
(2)  Parallel connection, R’p = Rx Ry  /  Rx + Ry = ….W.


RESULT:-
(1) Unknown resistance, Rx = ….W, Ry = ….W

Wheatstone Bridge Observation table
 

CONCLUSION :
It is possible to verify the rules for series and parallel connections with the help of a meter bridge.


Precautions:-
(1) Make all connections tight.
(2)  Clean the ends of the connecting wire with a sand paper.
(3)  Plug keys of resistance box should be very tight.
(4)   Adjust the value of R from the resistance box such that null-point is obtained between 40 cm. and 60 cm.

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