Electromagnet core used by James Joule, c. 1839 to measure electromagnetic attraction. Made up of 16 square sections of iron wires and covered in strips of cotton.

Deduced that the attractive force of two electromagnets for one another is directly proportional to the square of the electric force to which the iron is exposed; or if E is electric current, w the length of wire and M magnetic attraction, M=E squared x W squared.

Originally 'wrapped with 22 yards of covered wire' now missing.