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AimTo verify the laws of the combination of resistances using a meter bridge.
Apparatus RequiredMeter Bridge
Leclanche cell
Resistance Box
Galvanometer
Jockey
Sand Paper
Connecting wire
Two resistance wires
A set square
Theory(i) The resistance of a resistance wire or coil can be determined using the following formula:
R = (l / 100) * R1
Here, R1 is the resistance from the resistance box in the left gap, and l is the length of the meter bridge wire from zero ends up to the balance point.
(ii) The combined resistance of two resistors r1 and r2 connected in series can be calculated using the following formula:
Rs = r1 + r2
ProcedureClean the connecting wires with sandpaper to remove any insulating coating on them.
Set up the meter bridge circuit as per the given diagram.
Connect a resistance box to the jockey terminal and set it to a known value.
Adjust the length of the resistance wire until the galvanometer shows zero deflection.
Note down the length l1 of the wire on the left-hand side and the length l2 on the right-hand side of the galvanometer.
Repeat steps 3-5 for different values of resistance in the resistance box.
Using the values of l1 and l2, calculate the value of the unknown resistance using the formula:
R = (l2 / l1) * R1
Where R1 is the known resistance in the resistance box.
Verify the laws of combination of resistances, i.e., verify that the equivalent resistance of resistors in series is the sum of their individual resistances, and the equivalent resistance of resistors in parallel is given by:
1/Req = 1/R1 + 1/R2 + 1/R3 + …
Where R1, R2, R3, etc. are the individual resistances.
Repeat the experiment multiple times to ensure accurate results.
Observation and ResultThe galvanometer shows zero deflection when the length of the resistance wire on both sides of the jockey terminal is the same.
When the known resistance in the resistance box is changed, the balance length of the resistance wire also changes.
Using the values of l1 and l2, the unknown resistance can be calculated using the formula:
R = (l2 / l1) * R1
To verify the law of combination of resistances for resistors in series, three resistors of known values can be connected in series, and their equivalent resistance can be calculated using the formula:
Req = R1 + R2 + R3
The equivalent resistance can also be calculated using the meter bridge, and the two values can be compared to verify the law.
To verify the law of combination of resistances for resistors in parallel, three resistors of known values can be connected in parallel, and their equivalent resistance can be calculated using the formula:
1/Req = 1/R1 + 1/R2 + 1/R3
The equivalent resistance can also be calculated using the meter bridge, and the two values can be compared to verify the law.
The experiment shows that the laws of combination of resistances are valid and can be verified using a meter bridge.