题目

Michael was a farmer in Texas．His parents moved to the USA from Ireland in the 1940’s， and they became very rich．Michael decided to go back to Ireland to meet his old grandfather， who was still living on the same farm．     Michael booked his ticket，flew to Shannon airport，hired a car and drove to the small village where his parents had been born．He asked for directions to the Ohagan farm，and in the end he drove up a bumpy path．He was shocked by what he saw：a small field，and in the middle of it was an old house，which looked as if it was falling down．Animals were给你一把毫米刻度尺，一根细铜丝和一根铁钉，用什么方法可以较准确地测出铁钉的直径？写出实验步骤和相应的计算公式．