Poynting, John Henry (1852–1914)

English physicist, mathematician, and inventor. He devised an equation by which the rate of flow of electromagnetic energy (now called the Poynting vector) can be determined.

In 1891 he made an accurate measurement of Isaac Newton's gravitational constant.

Poynting was born near Manchester and studied there at Owens College, and at Cambridge. From 1880 he was professor of physics at Mason College, Birmingham (which became Birmingham University in 1900).

In On the Transfer of Energy in the Electromagnetic Field 1884, Poynting published the equation by which the magnitude and direction of the flow of electromagnetic energy can be determined. This equation is usually expressed as:

S = (1/μ)EB sin θ

where S is the Poynting vector, μ is the permeability of the medium, E is the electric field strength, B is the magnetic field strength, and θ is the angle between the vectors representing the electric and magnetic fields.

In 1903, he suggested the existence of an effect of the Sun's radiation that causes small particles orbiting the Sun to gradually approach it and eventually plunge in. This idea was later developed by US physicist Howard Percy Robertson (1903–1961) and is now known as the Poynting–Robertson effect. Poynting also devised a method for measuring the radiation pressure from a body; his method can be used to determine the absolute temperature of celestial objects.

Poynting's other work included a statistical analysis of changes in commodity prices on the stock exchange in 1884.