Experiment 3 : To verify the laws of combination (series) of resistances using a metre bridge.

Experiment 3 (a) : To verify the laws of combination (series) of resistances using a metre bridge.

Aim:

To verify the laws of combination (series) of Resistance using a Metre Bridge

Apparatus:

A metre bridge, a Leclanche cell (battery eliminator), a galvanometer, a resistance box, a jockey, two resistance wire or two resistance coils known resistances, a set square, sand paper and connecting wires.

Theory:

(i) The resistance (r) of a resistance wire or coil is given by 

$r = \frac{(100-l)R}{l}$

where R is the resistance from the resistance box in the left gap and l is the length of the metre bridge wire from zero end upto balance point.

(ii) When two resistance $r_1$ and $r_2$ are connected in series then their combined resistance

$R_S = r_1 + r_2$

Procedure :

1. Mark the two resistance coils as $r_1$ and $r_2$.
2. To find $r_1$ and $r_2$ proceed same way as in experiment 1.
3. Connect the two coils $r_1$ and $r_2$ in series as shown in figure in the right gap of metre bridge and find the resistance of this combination. Take at least three sets of observations.
4. Record your observation as fallow:

Observations :

Table for length (l) and unknown resistance (X).

Calculations : 

1. Theoretical value = $4.9\Omega [R_S = r_1+r_2 = 4.9\Omega]$
2. Experimental value = $5.83\Omega [R_S = 5.83]$
3. Difference = $0.9\Omega$

Result : 

Within limits of experimental error, experimental and theoretical values of $R_S$ are same. Hence, law of resistances in series is verified.

Precautions : 

1. The connections should be neat, clean and tight.
2. Thick copper wires should be used for the connections after removing the insulation near their ends by rubbing with sand paper.