This step consisted in taking the current induced in the revolving helix or armature (by the field magnets) and sending it back through the coils of the field magnets which produced it, thereby increasing the energy of the field magnet coils, and they in turn with an increased efficiency and reciprocal action induce still stronger currents in the armature coils, and so a building up process, or principle of mutual and reciprocal excitation, is carried on until the maximum efficiency is reached. This principle was the discovery of Soren Hjorth, of Copenhagen, and is fully described in his British patent, No. 806 of 1855, for “An Improved Magneto-Electric Battery.” As the prototype of the dynamo, it is worthy of illustration. In the illustration, Figs. 18 and 19, _a_ is a revolving wheel bearing the armature coils, _C_ permanent magnets, _d_ electro-magnets (field magnets), and _g_ the commutator. Quoting from his specifications, he says: “The permanent magnets acting on the armatures brought in succession between their poles, induce a current in the coils of the armatures, which current, after having been caused by the commutator to flow in one direction, passes round the electro-magnets (field magnets), charging the same and acting on the armatures. By the mutual action between the electro-magnets and the armatures an accelerating force is obtained, which in result produces electricity greater in quantity and intensity than has heretofore been obtained by similar means.” Although the principle of the dynamo was clearly embodied in the Hjorth patent, its value was not appreciated until some time later. Eleven years later Wilde (U. S. Pat. No. 59,738, Nov. 13, 1866), employed a small machine with permanent magnets to excite the coil-wound field magnets of a larger machine. But Siemens (British Pat. No. 261 of 1867), taking up the principle employed by Hjorth, dispensed with his superfluous permanent magnets, having found that the residual magnetism, which always remained in iron which has once been magnetized, was sufficient as a basis to start the building up process. Farmer, Wheatstone and Varley also recognized this fact about the same time. Siemens’ patent also was the first embodiment of what is known as the bobbin armature. Gramme and D’Ivernois (British Pat. 1,668 of 1870, and U. S. Pat. No. 120,057, of Oct. 17, 1871), were the first to bring out the continuously wound ring armature. Active development now began in various types and by various inventors, including Weston, Brush, Edison, Thomson and Houston, Westinghouse, and others, who have brought the dynamo to its present high efficiency.