The fact that a dynamo could be reversed and run as a motor was known probably as early as 1838, but the great value of this reversibility does not seem to have been realized until 1873. At an industrial exhibition held at Vienna in that year, it so happened that a workman or machinery attendant connected two cables to a dynamo which was standing idle, and he was much surprised to find that it at once began to revolve at a great speed. It was then seen that the cables led to another dynamo which was running, and that the current from this source had made the first dynamo into a motor. There are many versions of this story, but the important point in all is that this was the first occasion on which general attention was drawn to the possibilities of the electric motor. The practical advantages afforded by the electric motor are many and great. Once we have installed a sufficiently powerful dynamo and a steam or other engine to drive it, we can place motors just where they are required, either close to the dynamo or miles away, driving them simply by means of a connecting cable. In factories, motors can be placed close to the machines they are required to drive, anywhere in the building, thus doing away with all complicated and dangerous systems of shafting and belts. In many cases where it would be either utterly impossible or at least extremely inconvenient to use any form of steam, gas, or oil engine, electric motors can be employed without the slightest difficulty. In order to realize this, one only has to think of the positions in which electrically-driven ventilating fans are placed, or of the unpleasantly familiar electric drill of the dentist. An electric motor is small and compact, gives off no fumes and practically no heat, makes very little noise, is capable of running for very long periods at high speed and with the utmost steadiness, and requires extremely little attention. CHAPTER X ELECTRIC POWER STATIONS