John Hopkinson (27.07.1849-27.03.1898) was an English physicist and engineer and a Fellow of the Royal Society (1877). He was born in Manchester. He graduated from the universities of London (1870) and Cambridge (1871). He worked as an engineer in Birmingham, and from 1878 he was head of a laboratory and a teacher of electrical engineering at the University of London, becoming professor in 1881.
His research was in electromagnetism and electrical engineering (electric motors, dynamos, the design of transformers and of alternating-current systems, magnetic circuits, the transmission of electrical energy, hysteresis). He discovered the sharp rise in the magnetic permeability of ferromagnets in a weak magnetic field near the Curie point (the Hopkinson effect), and together with his brother E. Hopkinson he gave a formula for calculating magnetic circuits (the Hopkinsons' formula). He proposed the connection of motors in parallel. He constructed a theory of magnetic flux.