Aim
To determine the resistance per cm of a given wire by plotting a graph for potential difference versus current.
Apparatus/ Material Required
 A wire of unknown resistance
 Battery
 Voltmeter
 Milliammeter
 Rheostat
 Plug key
 Connecting wires
 Piece of sandpaper
Circuit Diagram
Theory
According to Ohm’s law, the electric current flowing through a conductor is directly proportional to the potential difference across its ends, provided the physical state (pressure, temperature, and dimensions) of the conductor remains unchanged.
If I is the current flowing through the conductor and V is the potential difference across its end, then
\(V\propto I\)and hence
V = RI
Where R is the constant of proportionality and is termed as the electrical resistance of the conductor. Resistance R depends on the dimensions and material of the conductor. The relationship between the resistance of a material and its length and area of the crosssection is given by the formula
\(R=\rho \frac{l}{A}\)’Where \(\rho\) is the specific resistance or resistivity and is a characteristic of the material of the wire.
Procedure
 Clean the ends of the connecting wire with the help of sandpaper to remove any insulating coating on them.
 Connect the resistance, rheostat, battery, key, voltmeter, and ammeter as shown in the figure.
 Make sure that the pointers in the voltmeter and milliammeter coincide with the zero mark on the measuring scale. If not, adjust the pointer to coincide with the zero mark by adjusting the screw provided at the base using a screwdriver.
 Note the range and the least count of the given voltmeter and milliammeter.
 Insert the key K and slide the rheostat to the end where the current flow is minimum.
 Note the voltmeter and the milliammeter reading.
 Remove the key K and allow the wire to cool. Again insert the key and slightly increase the voltage by moving the rheostat. Note down the milliammeter and voltmeter reading.
 Repeat step 7 for four different adjustments of the rheostat. Document the readings in a tabular column.
Observations
Range of ammeters = _____ mA to _____ mA
The Least count of ammeter = _____ mA
Range of voltmeter = _____ V to ____ V
The Least count of voltmeter = _____ V
The Least count of meterscale = _____ m
Length of the given wire, l = _____ m
S. No  The applied potential difference
(voltmeter reading V) 
Current flowing through the wire
(Milliammeter Reading A) 
Calculations

 Plot a graph between the potential difference across the wire V and the current I flowing through the wire as shown below.
2. Determine the slope of the graph. The resistance of the given wire is then equal to the
Reciprocal of the slope.
From the graph, R = BC/AB = _____ Ω
3. Resistance per unit length of the wire = R/t = _____ Ωm^{–1}
Error
\(\frac{\Delta R}{R}=\frac{\Delta V}{V}+\frac{\Delta I}{I}=….\)Here, R is the resistance per unit length and ΔR is the estimated error. ΔV
And ΔI are the least count of voltmeter and ammeter respectively.
Result
The potential difference across the wire varies linearly with the current.
The resistance per unit length of the wire is ( R ± ΔR) = _____ ± _____ Ωm^{–1}).
Viva Voice
1. State Ohm’s Law.
Answer:
Ohm’s law states that the potential difference across an ideal conductor is proportional to the current through it. The constant of proportional is known as the resistance R. Ohm’s law is given by V = IR.
2. Which are the factors on which the resistance of a conductor depends on?
Answer:
The resistance of a conductor depends on the following factors:
 Resistivity
 Length
 Temperature
 Crosssectional area
3. What is a rheostat?
Answer:
A rheostat is a variable resistance that is used to control the current.
4. What is the shape of a V vs I graph for a linear resistor?
Answer:
The shape of the V vs I graph for a linear resistor is a straight line.
5. What is the reciprocal of resistivity called?
Answer:
The reciprocal of resistivity is called conductivity.
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