Section – A Experiment -3
Objective : To verify the laws of combination (series) of resistances by using a meter bridge.
Required Apparatus:
A metre bridge, a Leclanche cell (battery eliminator), a galvanometer, a resistance box, a jockey, two resistance wires or two resistance coils of known resistances, a set square, connecting wires, a piece of sandpaper.
Formula Used:
(i) The resistance r of a resistance wire or coil is given by
Where, R is known resistance placed in the left gap and unknown resistance r in the right gap of the metre bridge. I cm is the length of a metre bridge wire from zero end up to the balance point.
(ii) When r1 and r2 are connected in series, then their combined resistance.
Circuit Diagram:
Observations Table:
Table for unknown resistance
Calculations:
The experimental value of Rs = 6.74 Ω
Theoretical value of Rs = r1 + r2 = 1.91 Ω + 4.79 Ω = 6.70 Ω
Difference ( if any ) = 6.74 Ω – 6.70 Ω = 0.04 Ω
Result:
Within the limits of experimental error, experimental and theoretical values of R are the same. Hence, the law of resistance in series is verified.
Precautions:
1. The connections should be neat, tight and clean.
2. Plugs should be tightly connected in the resistance box.
3. The movement of the jockey should be gentle and it shouldn’t be rubbed.
4. The key K should be inserted only when the observations are to be taken.
5. The null point should be between 30 cm and 70 cm.
6. To avoid the error of parallax, the set square should be used to note the null point.
7. There shouldn’t be any loops in the wire.
Sources Of Error:
1. The screws of the instrument might be loose.
2. The plugs may not be clean.
3. The wire might be of non-uniform diameter.