Faraday effect. A large electromagnet used by british scientist Michael Faraday (1791-1867) to demonstrate the Faraday effect. This is the rotation of the plane of polarisation of a beam of light due to the presence of a strong magnetic field. Faraday demonstrated this in 1845, with the light passing through 'heavy glass' (a lead borosilicate glass) resting on the poles of the magnet. This object is preserved at the Royal Institution, London.