Section A Experiment – 3
Objective
To verify the laws of combination (parallel) of resistances using a metre bridge.
Apparatus Required
A metre bridge, a Leclanche cell or battery eliminator, a galvanometer, a resistance box, a jockey, two resistance wires or two resistance coils known resistances, a set square, sand paper and connecting wires.
Formula Used
\frac{1}{R_{p}}=\frac{1}{R_{1}}+\frac{1}{R_{2}}
R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}
S=(\;\frac{100−l}{l}\;)\;R
Diagram
Here R1 = 1 Ω and R2 = 2 Ω
Calculation
R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}
R_{P}=\frac{1×2}{1+2}
R_{P}=\frac{2}{3}
R_{P}=0.66\;Ω
Result
Experiment value of parallel combination=RP= 0.60 Ω
The theoretical value of parallel combination,
R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}
R_{P}=\frac{R_{1}R_{2}}{R_{1}+R_{2}}
R_{P}=\;0.66\;Ω
Hence, laws of combination (parallel) of resistance is verified using a metre bridge.
Precautions
- All connections should be neat and tight.
- All the plugs in the resistance box should be tight.
- Clean the connecting wires and the connecting points of metre bridge properly with sandpaper.
- The jockey should be held perpendicular to wire of metre bridge.
- Try to obtain the balance point between 40 cm and 60 cm.
- Move the jockey gently on the wire and do not keep the jockey and the wire in contact for a long time.
Sources of errors
- The keys of the resistance box may not be clean and tight.
- The wire may not be uniformly thick throughout.
- The screws of the instrument may be loose.