John Kerr (physicist)

Rev. John Kerr (December 17, 1824 – August 15,1907) was a Scottish physicist who inaugurated the field of electro-optics.

Life

Born in Ardrossan to a fish merchant, Kerr attended the University of Glasgow from where he earned a master's degree in divinity, subsequently being ordained a minister of the Free Church of Scotland. He also displayed an enthusiasm and ability in mathematics and physics and when his contemporary Lord Kelvin became professor of natural philosophy in 1846 the two became close friends, Kerr helping Kelvin to found the first physical laboratory at a British university.Steele (2004)]

In 1857, Kerr became lecturer in mathematics and physical science at the Free Church Training College in Glasgow. It was there, in 1875, that he discovered the Kerr electro-optic effect. A strong electric potential, applied in a direction normal to a beam of light, causes a difference in refractive index for light polarised in the plane of the field and light polarised normal to it, an effect known as birefringence. This causes the resultant polarisation of the light to change.

The 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 in order to measure its speed. Earlier measurements had used mechanical means of modulation achieving frequencies of around 10 kHz while the Kerr cell allow frequencies of 10 MHz and greater precision of measurement. Kerr's original cell was a glass block while modern cells are more commonly filled with liquids such as nitrobenzene.

In the Kerr Effect, the change in refractive index is proportional to the square of the electric field. Where the relationship is linear, the effect is known as the Pockels effect. Kerr also demonstrated a similar effect for magnetic fields known as the magneto-optic Kerr effect. Light from lasers allows the achievement of the effect using the light's own electric field, the "AC Kerr effect".

Kerr was an early champion of the metric system in the UK.

He died in Glasgow.

Honours

*Honorary LL.D from the University of Glasgow in recognition of his contributions to physical science (1864);
*Fellow of the Royal Society (1890);
*Royal Medal of the Royal Society (1898);
*Civil list pension (1902).

References

Bibliography

*Obituaries:
**"The Times", August 19, 1907, 12"c"
**"Nature", 76 (1907), 575–6----
*Green, G. & Lloyd, J.T. (1970) "Kelvin's instruments and the Kelvin Museum" "pp"55–6
*Kerr, K. (1863) "The Metric System"
*— (1867) "An Elementary Treatise on Rational Mechanics"
*Steele, R. rev. Anita McConnell (2004) " [http://www.oxforddnb.com/view/article/34300 Kerr, John (1824-1907)] ", "Oxford Dictionary of National Biography", Oxford University Press, accessed 19 June 2005 ODNBsub