John Hopkinson, FRS, (27 July 1849 – 27 August 1898) was a British physicist, electrical engineer, Fellow of the Royal Society and President of the IEE (now the IET) twice in 1890 and 1896. He invented the three-wire (three-phase) system for the distribution of electrical power, for which he was granted a patent in 1882. He also worked in many areas of electromagnetism and electrostatics, and in 1890 was appointed professor of electrical engineering at King's College London, where he was also director of the Siemens Laboratory. Hopkinson's law, the magnetic counterpart to Ohm's law, is named after him. Life and career John Hopkinson was born in Manchester, the eldest of 5 children. His father, also called John, was a mechanical engineer. He was educated at Queenwood School in Hampshire and Owens College in Manchester. He won a scholarship to Trinity College, Cambridge in 1867 and graduated in 1871 as Senior Wrangler, having placed first in the demanding Cambridge Mathematical Tripos examination. During this time he also studied for and passed the examination for a BSc from the University of London. Hopkinson could have followed a purely academic career but instead chose engineering as his vocation. He was a Cambridge Apostle. After working first in his father's engineering works, Hopkinson took a position in 1872 as an engineering manager in the lighthouse engineering department of Chance Brothers and Company in Smethwick. In 1877 Hopkinson was elected a Fellow of the Royal Society in recognition of his application of Maxwell's theory of electromagnetism to problems of electrostatic capacity and residual charge. In 1878 he moved to London to work as a consulting engineer, focusing particularly on developing his ideas about how to improve the design and efficiency of dynamos. Hopkinson's most important contribution was his three-wire distribution system, patented in 1882. In 1883 Hopkinson showed mathematically that it was possible to connect two alternating current dynamos in parallel-—a problem that had long bedevilled electrical engineers. He also studied magnetic permeability at high temperature, and discovered what was later called the Hopkinson peak effect. The series-parallel method of electric motor control, for which Hopkinson was granted a British patent in 1881, would prove to be an important advance in the development of electric railways. He applied for a US patent in 1892, triggering an interference proceeding against American inventor Rudolph M Hunter, who had been granted a US patent for the method