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 The success of the meter, however, was soon eclipsed by that of the great invention with which the names of William and Frederick Siemens must always be connected—the regenerative furnace. The brothers had long endeavoured to apply the principle of their condenser to various manufacturing processes, especially to those in which, as in salt-making, large quantities of liquid have to be evaporated. Their efforts met with little practical success until they finally hit on the very simple idea of applying the principle of the condenser to furnaces, an idea which was embodied in a patent taken out by Frederick in 1856. The products of combustion from the furnace, instead of passing direct to the chimney, were led through a chamber filled with refractory brickwork, to which they gave up their heat. As soon as the chamber was sufficiently hot, the current was shut off, and the air-supply of the furnace led through it. The air thus reached the burning fuel hot, instead of cold. By the use of two chambers used alternately to receive and give out the heat, the process was made continuous. By a further improvement gas was used in place of solid fuel, and this was passed through the ‘regenerator’ so that it arrived at the place of combustion in a highly heated state. Not only was there an enormous economy of heat, but the gas could be made from fuel of a very inferior sort, while processes could be conducted in the open furnace which could only be carried out in crucibles when solid fuel was employed. The first practical application of the furnace was to the melting and reheating of steel in 1857. It was soon after applied, in a modified form, to heating the air for blast furnaces, then to glass-making (the subject of the last lecture ever given by Faraday was the use of the furnace at Chance's glass-works), and eventually to a large number of industrial processes where great heat is required. Its latest and most important application was to the manufacture of steel either by melting wrought iron and cast iron together on the open hearth of the furnace or direct from iron ore. The former was known as the ‘Siemens-Martin’ process, and the latter as the ‘Siemens’ or ‘ore’ process. For the manufacture of steel the ‘Siemens’ process was first used in 1865 or 1866; its employment spread rapidly, so that by 1882 it was estimated that four million tons of steel had been produced by it, and in 1896 the output for the whole world was calculated at over seven million tons as compared with over eleven million tons produced by the Bessemer process. In Great Britain 2,355,000 tons were produced in 1896 by the Siemens process, and 1,845,000 by the Bessemer process.

In order to develop the process, and to test its working on a large scale, Siemens with some personal friends formed a company, and works were established at Landore in South Wales in 1869. Though for a time the company promised well and held a leading place among the steel-works of the kingdom, it was not commercially successful, and the attempt was abandoned about 1888.

Siemens had been naturalised as an Englishman in 1859, and he obtained medals at the exhibitions of London in 1862 and of Paris in 1867. In 1860 he became a member of the Institution of Civil Engineers, in special recognition of his merits—since he was a manufacturer rather than an engineer—and in 1862 he was elected a fellow of the Royal Society.

Meantime he had turned his attention to electrical science, and was building up another reputation. On the first introduction of the electric telegraph, Werner Siemens appreciated its possibilities, and determined to devote himself to its development. In 1847 he associated himself with Halske and founded in Berlin the great firm of Siemens & Halske, of which William was appointed the London agent. Werner discovered the method of insulating telegraph-wires with gutta-percha, and the use of such wires for conveying messages under water led to the invention of submarine cables. The first of these was the Dover to Calais cable, laid, after an unsuccessful attempt in the previous year, in 1851. This was soon followed by others, with many of which the firm of Siemens & Halske was associated. In 1858 this department of their business had developed to such an extent that the brothers determined to establish works in England, and a small factory was started in Millbank, afterwards in 1866 transferred to Charlton in Kent, where the great works of Siemens Brothers (Werner, William, and Carl) were eventually established. These works in course of time occupied an area of above six acres, and employed over two thousand hands. Of the undertakings carried out by the brothers, one of the greatest was the telegraph line from Prussia to Teheran, a length of 2,750 miles, which formed a principal part of the direct line from England to India. This was carried out by the London and Berlin firms jointly in 1869. A few years later a still more important undertaking was brought to a successful issue by the London firm alone, which in 1874 laid the direct Atlantic cable. For this the cable-ship Faraday was specially