Class 12 Physics practical readingTo verify the laws of combination of resistances (series and parallel) using a metre bridge.

Aim

To verify the laws of combination of resistances (series and parallel) using a metre bridge.

Material Required

A metre bridge, a sensitive galvanometer, two different resistances

(carbon or wire-wound resistors), a resistance box, a jockey, a rheostat,

a plug key, a cell or battery eliminator, thick connecting wires and a

piece of sand paper.

Theory

When two resistances R1

 and R2 are connected in series, the resistance

of the combination Rs is given by

Rs = R1 + R2

When connected in parallel, the resistance Rp of the combination is

given by

1/Rp = 1/R1 + 1/R2

Circuit diagram for series connection

Circuit diagram for parallel connection

Procedure

1. Set up the circuit as shown in Fig. E 

2. Tighten all plugs in the resistance box (RBOX) by rotating and

pressing each plug to ensure that all plugs make good electrical

contacts. Clean the ends of connecting wires using a sand paper

before making the connections.

3. Remove some plug(s) from the resistance box to get suitable

value of resistance R. Obtain the null point D on the metre bridge

wire by sliding the jockey between ends A and C as was done in

Experiment 2.

4. Note resistance R and lengths AD and DC in the observation

table.

5. Calculate the experimental value of the equivalent series

resistance (X) of combination of resistances as shown in

Table E 3.1.

6. Repeat the experiment for four more values of resistances R.

Obtain the mean value of unknown resistance.

7. Repeat steps 2 - 6 by connecting resistances R1 and R2 in parallel

as shown in Fig. E 3.2 and calculate the experimental value of

the equivalent parallel resistance (X) of combination of resistance.

Observation & Calculations

Precautions

1. All the connections and plugs should be tight.

2. Jockey should be moved gently over the metre bridge wire.

3. Plug keys of the resistance box should be made tight by rotating

it in clockwise direction.

4. Null points should be in the central region of the wire

(30 cm to 70 cm).

Sources of error

1. The wires may not have uniform thickness. 

2. All plugs in the resistance box may not be tight. 

3. Heating of wire may take place due to continuous flow of current. 

4. Development of contact resistance due to improper connections.