Heinrich Daniel Ruhmkorff (Rühmkorff) (15 January 1803 in Hanover – 20 December 1877 in Paris) was a German instrument maker who commercialised the induction coil (often referred to as the Ruhmkorff coil.)
Ruhmkorff was born in Hanover. He changed the "ü" to "u" in his name when living abroad. After an apprenticeship with a German mechanic, he moved to England. Biographies say that he worked with the inventor Joseph Bramah, but this is unlikely since Bramah died in 1814. He may, though, have worked for the Bramah company. In 1855, he set up a shop in Paris, where he gained a reputation for the high quality of his electrical apparatus.
Although Ruhmkorff is often credited with the invention of the induction coil, it was in fact invented by Nicholas Callan in 1836. Ruhmkorff's first coil, which he patented in 1851, utilized long windings of copper wire to achieve a spark of approximately 2 inches (50 mm) in length. In 1857, after examining a greatly improved version made by an American inventor, Edward Samuel Ritchie, Ruhmkorff improved his design (as did other engineers), using glass insulation and other innovations to allow the production of sparks more than 30 centimetres long. Ruhmkorff patented the first version of his induction coil in 1851, and its success was such that in 1858 he becomes the first recipient of the Volta Prize, 50,000 French francs award by Napoleon III for one of the most important discoveries in the application of electricity. He died in Paris in 1877.
The "Ruhmkorff" lamp
In several of Jules Verne's science-fiction novels, so-called "Ruhmkorff lamps" are mentioned. These were an early form of portable electric lamp. The lamp consisted of a Geissler tube that was excited by a battery-powered Ruhmkorff induction coil. Initially the lamp generated white light by using a Geissler tube filled with carbon dioxide. However, the carbon dioxide tended to break down. Hence in later lamps, the Geissler tube was filled with nitrogen (which generated red light), and the glass was replaced with glass containing uranium salts (which fluoresced with a green light).
Intended for use by miners, the lamp was actually developed both by Alphonse Dumas, an engineer at the iron mines of Saint-Priest and of Lac, near Privas, in the départment of Ardèche, France, and by Dr. Camille Benoît, a medical doctor in Privas. In 1864, the French Academy of Sciences awarded Dumas and Benoît a prize of 1,000 francs for their invention.
This lamp could be considered as a predecessor of modern fluorescent lanterns, because, as in the "Ruhmkorff lamp", modern portable fluorescent lamps use an inverter (oscillator + step-up transformer) to convert low voltage DC current from dry cells or storage batteries into AC or even pulsating current at a voltage high enough as to ionize the gas in the fluorescent tube and thus power it at the required nominal level. Modern portable fluorescent lamps contains neither nitrogen, nor CO2, nor uranium glass; instead a modern fluorescent tube is filled with argon at a very low pressure, along with a few milligrams of mercury. The lamp electrodes usually consists of a double or triply coiled tungsten wire covered with substances, such as alkaline metal oxides, that readily emit free electrons, aiding the ionization of the gas and thereby initiating and sustaining the electric arc. The inner walls of the tube are coated with a thin layer of fluorescent substances, which, when they are excited by the 253.7 nanometres (9.99×10 in) line of UV radiation from a mercury arc, emits visible light, usually in the range of 6500K (daylight) colour temperature. The spectral characteristics (coloration) of such a light depends entirely on the chemical nature of those "phosphors".
Modern inverters for portable fluorescent lamps do not rely on electromechanical and electromagnetic vibrating switching contacts to produce the required current interruptions in the transformer’s primary winding that are required in order to produce the induced high voltages in the transformer's secondary winding, as occurs in Ruhmkorff coils; instead, the primary circuit of the transformer is switched by means of one or two transistors oscillating at frequencies of tens or even hundreds of thousand cycles per second, which results in smaller and lighter devices having a very good efficiency in lumens/watt for a given battery consumption or a longer battery charge life for a given light flux when compared with incandescent lamps. Additionally, such high frequencies virtually eliminates the stroboscopic effect which is very noticeable in those cases when tubes are powered by means of low frequency alternating or pulsating currents as happens when regular 50 or 60 Hz AC mains are used as power source. Transistorized devices are also silent, unlike true Ruhmkorff coils, which make its characteristic buzzing noises when in action, even if they are contained in closed casings or boxes. Indeed, the principle of modern fluorescent portable lamps or lanterns remains the same as in Dumas and Benoit's original mining electric lamp.
The asteroid 15273 Ruhmkorff, discovered in 1991 by E. W. Elst, is named after Heinrich Daniel Ruhmkorff.