Determine the diameters dA and dB.

To determine the diameters $d_A$ and $d_B$ of wires A and B, you would use a micrometer screw gauge.

• Place the wire between the anvil and spindle of the micrometer.
• Turn the thimble until the ratchet clicks, indicating the wire is gently gripped.
• Read the main scale and thimble scale readings.
• Record the observed diameter.
• Repeat this measurement at least three to five times at different points along the wire and at different orientations to account for any irregularities.
• Calculate the average of these readings to obtain the diameter $d_A$ (or $d_B$).
• Remember to check for and apply any zero error correction to your readings.

Once you have determined the diameter $d$ (either $d_A$ or $d_B$) from Step 1, you can calculate $r$.
The formula given is:

$$r = \left(\frac{d}{2}\right)^2$$

Here, $d$ is the measured diameter of the wire, and $\frac{d}{2}$ represents the radius of the wire. Therefore, $r$ is the square of the radius of the wire.
For example, if you measured $d_A = 0.30 \text{ mm}$, then:

$$r_A = \left(\frac{0.30 \text{ mm}}{2}\right)^2 = (0.15 \text{ mm})^2 = 0.0225 \text{ mm}^2$$

You would perform this calculation for both wires A and B using their respective diameters.

The remaining parts (iii) through (xv) describe the experimental procedure for setting up circuits, taking current and voltage readings, tabulating results, and plotting graphs to determine the resistance of the wires. These are instructions for conducting the experiment and analyzing the data you would collect.