Ruhmkorff induction coil, 19th century artwork. This electromagnetic induction coil was invented in 1851, by the German engineer Heinrich Daniel Ruhmkorff. When connected to sparking electrodes in an evacuated chamber (centre left), the coil could produce high- voltage electrical sparks over a 30-centimetre-wide gap. The technology was used in the first radio transmitters and in the Crookes and Geissler tubes. A later version was developed into the first alternating-current transformer. Artwork from The Circle of Sciences, London, 1862- 67.