Kerr, John (1824–1907)

Scottish physicist who discovered the Kerr effect, which produces double refraction in certain media on the application of an electric field.

Kerr was born in Ardrossan, Ayrshire, and studied at Glasgow. He was lecturer in mathematics at the Free Church Training College for Teachers, Glasgow, 1857–1901, and set up a modest laboratory there.

In 1875 Kerr demonstrated that double refraction occurs in glass and other insulators when subjected to an intense electric field. The effect was strongest, he found, when the plane of polarization was 45° to the field, and zero when perpendicular or parallel. Kerr extended the work to other materials, and constructed cells in which he could study liquids such as carbon disulphide and paraffin oil. He showed that the extent of the effect, more precisely called the electro-optical Kerr effect, is proportional to the square of the field strength.

In 1876 he demonstrated what is now known as the magneto-optical Kerr effect. A beam of plane-polarized light was reflected from the polished pole of an electromagnet. The beam became elliptically polarized, when the magnet was switched on. The effect depended on the position of the reflecting surface with respect to the direction of magnetization and to the plane of incidence of the light.