A purely local experiment - Poynting and the mean density of the Earth

These days John Henry Poynting is best known for his association with the Poynting vector, which describes the flow of energy in an electromagnetic field, and little is known of his life or work. Yet in the 1890s he caught the popular imagination as 'the man who weighed the Earth'. His experiment, using a novel method with a common balance, was part of a heroic tradition, gained him Cambridge University's Adams Prize, and set new standards of precision. Yet in performing this experiment, Poynting seemed to step outside the traditions of late nineteenth century Cambridge University where he was educated, and it is probably significant that he was a Unitarian throughout his life. This paper examines Poynting's experiment and his interpretation of it in the context of the rest of his life and work.