Electromagnet used by James Joule in about 1839 for experiments on the attracting power of magnets

Electromagnet used by James Joule in about 1839 for experiments on the attracting power of magnets. Iron bar is wound with '88 yards of covered copper wire 1/16" diameter.'

One of a pair of magnets used to test the law that 'the attractive force of two electromagnets is directly proportional to the square of the electric force to which the iron is exposed', i.e. M (attraction) = E squared (electric current) x W squared (length of wire). Results showed that as the length of the bar is increased, so the power of attraction increases.