The electric railway was made possible by the invention of the dynamo and the discovery that the dynamo is reversible. At the Industrial Exposition in Berlin in 1879 there was exhibited the first practical electric locomotive, the invention of Doctor Siemens. The locomotive and its passenger-coach were absurdly small. The track was circular, and about one thousand feet in length. This diminutive railway was referred to by an American magazine as "Siemens' electrical merry-go-round." But the electrical merry-go-round aroused great interest because of the possibilities it represented (Fig. 54). [Illustration: FIG. 54--FIRST ELECTRIC LOCOMOTIVE] The current was generated by a dynamo in Machinery Hall, this dynamo being run by a steam-engine. An exactly similar dynamo mounted on wheels formed the locomotive. The current from the dynamo in Machinery Hall was used to run the other as a motor and so propel the car. The rails served to conduct the current. A third rail in the middle of the track was connected to one pole of the dynamo and the two outer rails to the other pole. A small trolley wheel made contact with the third rail. The rails were not insulated, but it was found that the leakage current was very small, even when the ground was wet. The success of this experiment aroused great interest, not only in Germany, but in Europe and America. America's greatest inventor, Edison, took up the problem. Edison employed no trolley line or third rail, but only the two rails of the track as conductors, sending the current out through one rail and back through the other. Of course, this meant that the wheels must be insulated, so that the current could flow from one rail to the other only through the coils of the motor. As in Siemens' experiment, the motor was of the same construction as the dynamo. The rails were not insulated, and it was found that, even when the track was wet, the loss of electric current was not more than 5 per cent. Edison found that he could realize in his motor 70 per cent. of the power applied to the dynamo, whereas the German inventor was able to realize only 60 per cent. The improvement was largely due to the improved winding. Edison was the first to use in practical work the compound-wound dynamo, and this was done in connection with his electric railway. Fig. 55 shows Edison's first electric locomotive. [Illustration: FIG. 55--FIRST EDISON ELECTRIC LOCOMOTIVE]