The size of dynamos or electrical generators grew rapidly. Faraday's original apparatus involved only about 71/4 lb of copper but the armature of a machine built by H. Wilde in 1866 required 576 lb of copper strip and wire (Fig. 41). This machine, which was belt-driven, was described at the time as 'a machine of enormous and unprecedented power', with an output so enormous as to melt 15 inches of No. II copper wire'. By comparison, a modern 500 MW turbo-generator operates at several hundred times the voltage of Wilde's machine and requires about 14 tons of copper for various components in the machine.Contributions to the general development of the dynamo were made by Field, Edison, Sprague and others; whilst a particularly important advance in armature winding was made by Z. T. Gramme, who in 1870 invented (or rather reinvented, following Pacinotti in 1860) the toothed-ring winding system for armatures. His armature consisted of a ring of iron wound over with an endless coil of wire and connected to a commutator consisting of copper bars insulated from each other. Gramme dynamos were then soon made on the self-exciting principle. In 1873 it was discovered at Vienna that a dynamo of the Gramme type could also act as an electric motor and one was set in rotation when a current was passed into it from a similar machine. 'Henceforth the electric transmission of power came within the possibilities of engineering.' (27)
The brothers Siemens exhibited an electric tramcar in Paris in 1881. By using transformers, L. Gaulard in 1883 proposed to employ high pressure alternating currents for the distribution of electricity over wider areas. Polyphase alternators and the long-distance transmission of electrical power by polyphase electrical currents were both first exhibited at the Frankfurt Exhibition in 1891.
In addition to those scientists already named, outstanding men in the 'heroic age' of electrical engineering included J. Hopkinson, Crompton, S. P. Thompson, Mordey and Ferranti, with the combined theoretical and practical genius of Kelvin behind them all.
These developments led to a tremendous increase in the supply of electric power; but demand, as today, soon threatened to outstrip supply. The introduction of power distribution systems for electricity supply, the installation of land and submarine telegraph cables, and the use of electric traction for trains were among the major consequences of the rapid progress in electrical engineering during the latter half of the 19th Century.