To measure the resistivity of a wire

Apparatus

Length of wire (nichrome, manganin), micrometer, ohmmeter, metre stick.

Experimental setup diagram

Note the resistance of the leads when the crocodile clips are connected together.
Tie a length (2 or 3 metres) of nichrome/manganin between the bars of the two stands as shown above. Stretch the wire enough to remove any kinks or 'slack' in the wire.
Connect the crocodile clips to the wire some distance l apart. Read the resistance of the leads plus the resistance of wire between the crocodile clips from the ohmmeter. Subtract the resistance of the leads to get the resistance R of the wire.
Measure the length l of the wire between the crocodile clips, with the metre stick or tape.
Increase the distance between the crocodile clips. Measure the new values of R and l.
Make a note of the zero error on the micrometer. Use the micrometer to find the diameter of the wire at different points, taking the zero error into account. Find the average value of the diameter d.
Calculate the resistivity $\rho = \frac{R}{l} A$ , where $A = \frac{\pi d^2}{4}$ .
Repeat this procedure for a number of different lengths.
Calculate the average value for $\rho$ .

Experimental Setup

Micrometer Reading/mm

Resistance of leads =
Micrometer zero error =
Average of micrometer readings =
Diameter of wire =

Average value of $\rho$ =

chatImportant

Safety glasses should be worn as the wire could snap when stretched.

infoNote

Use a micrometer with a slip-screw.

infoNote

If clamps are unavailable, two students may hold the stands to keep the wire stretched enough to avoid kinks.

infoNote

Alternatively stretch the wire between two nails, which are positioned one to two metres apart on a piece of wood.

infoNote

The resistivity of nichrome is 100 × 10 $^{-8}$ $\Omega$ m (at 20° C).

infoNote

The resistivity of manganin is 48 × 10 $^{-8}$ $\Omega$ m (at 20 °C).

infoNote

The resistivity values given depends on composition of the alloys used.