Invented by Langmuir in 1926, this device produces a temperature of 3700 degrees centigrade (6692 degrees Fahrenheit). Tungsten can be melted, diamond vapourised.
A jet of hydrogen gas is dissociated as it passes through an electric arc. H2 > H + H - 422 kJ. An endothermic reaction, with the intensely hot plasma core of the arc providing the dissociation energy. The atomic hydrogen produced soon recombines; and this recombination is the source of such high temperatures (easily outperforming oxy-hydrogen: 2800oC and oxy-acetylene: 3315oC).
The hydrogen can be thought of as simply a transport mechanism to extract energy from the arc plasma and transfer it to a work surface. It produces a true flame, as the heat is liberated by a chemical reaction. H + H > H2 + 422kJ. The molecular hydrogen burns off in the atmosphere, contributing little to the heat output.
From the May 1, 1926 issue of The Science News-Letter -
"... developed by Dr. Irving Langmuir, assistant director of the [General Electric Company] Schenectady, [N.Y.], laboratory, and makes use of what he calls flames of atomic hydrogen .... Electric currents of 20 amperes and at voltages ranging from 300 to 800"
From A Text Book of Inorganic Chemistry, Partington 1946 -
"Atomic hydrogen. - Langmuir (1912) has shown that hydrogen in contact with a tungsten wire heated by an electric current at low pressure, is dissociated into atoms: H2 <=> 2H.
This splitting of the hydrogen molecule and subsequent immediate re-combination is attended by the radiation of a large amount of heat energy, about 100,00 calories per gram molecule. [30.7 kwh/Liter of H2 gas] This energy amplification is manifested as a glowing white-hot ball of heat.
The atomic hydrogen so formed is chemically very active. Langmuir also showed that atomic hydrogen is formed when an electric arc between tungsten electrodes is allowed to burn in hydrogen at atmospheric pressure.
The atomic hydrogen was blown out of the arc by a jet of molecular hydrogen directed across the arc, and formed an intensely hot flame as a result of H atoms recombining into H2 molecules, the reaction resulting from the recombination of H atoms, causing a huge release of heat energy, is capable of melting tungsten (m.p. 3400oC [61520F]). This "recombinant" flame obtains its heat not from combustion but from the result of recombining hydrogen atoms into H2 molecules. [edits & emphasis added]
It is suitable for melting and welding many metals. Iron can be melted without contamination with carbon, oxygen or nitrogen. Because of the powerful reducing action of the atomic hydrogen, alloys can be melted without fluxes and without surface oxidation. A feature of the flame is the great rapidity with which heat can be delivered to a surface, which is very important in welding operations."
Used extensively before the second world war, particularly in Germany. The method was known as Arc-Atom. The welding torch consists of two inclined tungsten electrodes across which an AC arc is struck. Annular nozzles around the electrodes carry hydrogen gas. The gas streams converge, forming a fan shaped flame. A transformer with an open circuit voltage of 300V is required to strike and maintain the arc in hydrogen.
Because of the high thermal conductivity of hydrogen, the plasma channel in the arc is constricted, offering very high energy concentration. Arc-Atom welding was of particular use for such jobs as surfacing dies, where the high flame temperature enabled a thin surface layer to be deposited on the thick base metal. Modern shielding gas methods such as MIG and TIG processes have replaced Arc-Atom, but do not have the extreme temperatures and arc constriction qualities of atomic hydrogen.