[Illustration: =DYNAMO= =MOTOR= Fig. 1. Diagram to illustrate the essential identity of the dynamo and the motor. The dynamo generates electricity when the armature or group of coils is forcibly revolved close to magnets, thus converting mechanical energy into electrical energy. The motor causes its armature to revolve forcibly when current is supplied to it from the dynamo. Thus the motor converts electrical energy into mechanical energy.] The essential step towards the commercial plane was taken when an efficient means was devised for transforming mechanical into electrical energy on a large scale. The first 'dynamo-electric' machines, invented about the middle of last century, were merely hand machines. Their power was limited by the strength of the permanent magnets employed in their construction; and although an increase in power was obtained by multiplying the number of magnets and driving by steam power, it was not sufficient for commercial purposes. In 1867 electro-magnets were first employed by Siemens and Wheatstone; and from this application there was developed a machine whose power as a generator of electricity was limited only by its size and the speed at which it was run. It is unnecessary for our present purpose to enter into the technical details of the modern electric generator and the modern electric motor. The principles underlying them are quite simple, although the theory of their design and the practice of their construction and operation are almost a science in themselves. A dynamo or electric generator is a machine for transforming mechanical into electrical energy; an electric motor is a machine for transforming electrical energy into mechanical energy. If, therefore, we place an electric motor upon a vehicle and supply it continuously with current from a dynamo, the motor will rotate and can be used to propel the vehicle. That is the essential mechanism of electric traction. The simplicity of the arrangement is enhanced by the fact that the dynamo and the motor are virtually the same machine. In the dynamo, a cylindrical 'armature' of coils is forced to rotate close to the poles of electro-magnets; the energy exerted in turning the armature against the influence of the electro-magnets is transformed into the energy of electric currents in the coils of the armature. In the motor, which also consists of an armature close to the poles of electro-magnets, the process is reversed. When a current is passed through the coils of the armature, the reaction between these currents and the electro-magnets causes the armature to revolve.