History
Michael Faraday discovered the principle of induction, Faraday's induction law, in 1831 and did the first experiments with induction between coils of wire. The induction coil was invented by the Irish scientist and Catholic priest Nicholas Callan in 1836 at the St. Patrick's College, Maynooth and improved by William Sturgeon and Charles Grafton Page. The early coils had hand cranked interrupters, invented by Callan and Antoine Philibert Masson (1806-1860). The automatic 'hammer' interrupter was invented by Christian Ernst Neeff (1782-1849) and Johann Philipp Wagner (1799-1879), and by the Rev. Prof. James William MacGauley (1806-1867) of Dublin, Ireland. Hippolyte Fizeau introduced the use of the quenching capacitor. Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary, in some coils using 5 or 6 miles (10 km) of wire. In the early 1850s, after examining an example of a Ruhmkorff coil, which produced a small spark of around 2 inches (50 mm) when energized, American inventor Edward Samuel Ritchie perceived that it could be made more efficient and produce a stronger spark by redesigning and improving its secondary insulation. His own design divided the coil into sections, each properly insulated from each other. Ritchie's induction coil proved superior to other designs of the day, initially producing a spark of 10 inches (25 cm) in length; later versions could produce an electrical bolt 24 inches (60 cm) or longer in length. The full story of Page's invention of the induction coil in its modern guise is told in Robert Post, "Physics, Patents, and Politics: A Biography of Charles Grafton Page" (Science History Publications, 1976. In 1857, one of Ritchie's induction coils was exhibited in Dublin, Ireland at a conference of the British Association, and later at the University of Edinburgh in Scotland. Ruhmkorff himself purchased a Ritchie induction coil, utilizing its improvements in his own work. Callan's induction coil was named an IEEE Milestone in 2006.
Induction coils were used to provide high voltage for early gas discharge and Crookes tubes and other high voltage research. They were also used to provide entertainment (lighting Geissler tubes, for example) and to drive small "shocking coils", Tesla coils and violet ray devices used in quack medicine. They were used by Hertz to demonstrate the existence of electromagnetic waves, as predicted by James Maxwell and by Lodge and Marconi in the first research into radio waves. Their largest industrial use was probably in early wireless telegraphy spark-gap radio transmitters and to power early cold cathode x-ray tubes from the 1890s to the 1920s, after which they were supplanted in both these applications by AC transformers and vacuum tubes. However their largest use was as the ignition coil or spark coil in the ignition system of internal combustion engines, where they are still used, although the interrupter contacts are now replaced by solid state switches. A smaller version is used to trigger the flash tubes used in cameras and strobe lights.
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