How to measure the diameter of a thin copper wire with length L The tool is a round pencil rod

Wrap the thin copper wire round the pencil, measure the coil length a, count the number of turns B, and use a / b

Why is the diameter of a thin copper wire measured in this way?
"First, measure the length of the copper wire with a ruler, then use the copper wire to wind several circles on the pencil. After winding the copper wire, divide the length of the copper wire by the number of circles to get the diameter of the copper wire"? I personally think that the calculation is the circumference of the bottom of the pen. If I answer well, I will add points
I have seen from the reference books that when measuring, we should wind the copper wire around the pen and do not take it down

It's the circumference of the cross section of the pen, not the diameter of the thin copper wire

After several hundred laps, measure the length with a micrometer and divide the length by the number of laps to get the diameter