Gramme, in 1871, modified the ring armature, and constructed the first machine, in which he made use of the Gramme ring and the dynamic principle. In 1872, Hefner-Alteneck, of the firm of Siemens and Halske, constructed a machine in which the Gramme ring is replaced by a drum armature, that is to say, by a cylinder round which wire is wound. ... Either the Pacinotti-Gramme ring armature, or the Hefner-Alteneck drum armature, is now adopted by nearly all constructors of dynamo-electric machines, the parts varying of course in minor details." The history of the dynamo since has been one of a gradual perfection of parts, resulting in the production of a great number of types, which can not here even be mentioned. _A. R. von Urbanitzky, Electricity in the Service of Man, pages 227-242._ _S. P. Thompson, Dynamo Electrical Machines._ ELECTRICITY: Electric Motors. It has been known for forty years that every form of electric motor which operated on the principle of mutual mechanical force between a magnet and a conducting wire or coil could also be made to act as a generator of induced currents by the reverse operation of producing the motion mechanically. And when, starting from the researches of Siemens, Wilde, Nollet, Holmes and Gramme, the modern forms of magneto-electric and dynamo-electric machines began to come into commercial use, it was discovered that any one of the modern machines designed as a generator of currents constituted a far more efficient electric motor than any of the previous forms which had been designed specially as motors. {775} It required no new discovery of the law of reversibility to enable the electrician to understand this; but to convince the world required actual experiment." _A. Guillemin, Electricity and Magnetism, part 2, chapter 10, section 3._ ELECTRICITY: A. D. 1835-1889. The Electric Railway.