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.
He died in Glasgow.