John Kerr was born on 17 December 1824 in
Ardrossan, Scotland. He was a student in Glasgow from 1841 to 1846, and at the Theological College of the Free Church of Scotland, in Edinburgh, in 1849. Starting in 1857 he was a mathematical lecturer at the
Free Church Training College in Glasgow. He died in
Glasgow in 1907. Kerr's most important experimental work was the discovery of double refraction in solid and liquid dielectrics in an electrostatic field (1875) which is now known as the
Kerr effect. In the Kerr effect, the difference between
refractive index experienced by an ordinary and extraordinary ray is proportional to the square of the
electric field. Where the relationship is linear, the effect is known as the
Pockels effect. Intense light from
lasers allows the achievement of the effect using the light's own electric field, the
AC Kerr effect. The Kerr effect is exploited in the
Kerr cell, which is used in applications such as
shutters in high-speed photography, with shutter-speeds as fast as 100 ns. In 1928 Karolus & Mittelstaedt used a Kerr cell to
modulate a beam of light to measure its
speed. Earlier measurements had used mechanical means of
modulation achieving
frequencies of around 10
kHz, but the Kerr cell allows frequencies of 10
MHz and greater precision of measurement. Kerr's original cell was a glass block. Modern cells are more commonly filled with liquids such as
nitrobenzene. Kerr also was an early champion of the
metric system in the UK. ==Honours==