The foregoing remarks apply to continuous current dynamos only. Alternating current can be used for exciting electro-magnets, but in this case the magnetic field produced is alternating also, so that each pole of the magnet has north and south magnetism alternately. This will not do for dynamo field magnets, and therefore an alternating current dynamo cannot feed its own magnets. The electro-magnets in such dynamos are supplied with current from a separate continuous current dynamo, which may be of quite small size. It is a very interesting fact that electric current can be generated by a dynamo in which the earth itself is used to provide the magnetic field, no permanent or electro-magnets being used at all. A simple form of dynamo of this kind consists of a rectangular loop of copper wire rotating about an axis pointing east and west, so that the loop cuts the lines of force of the Earth’s magnetic field. The dynamo provides us with a constant supply of electric current, but this current is no use unless we can make it do work for us. If we reverse the usual order of things in regard to a dynamo, and supply the machine with current instead of mechanical power, we find that the armature begins to revolve rapidly, and the machine is no longer a dynamo, but has become an electric motor. This shows us that an electric motor is simply a dynamo reversed. Let us suppose that we wish to use the dynamo in Fig. 20 as a motor. In order to supply the current we will take away the lamp and substitute a second continuous-current dynamo. We know from Chapter VII. that when a current is sent through a coil of wire the coil becomes a magnet with a north and a south pole. The coil in our dynamo becomes a magnet as soon as the current is switched on, and the attraction between its poles and the opposite poles of the magnet causes it to make half a revolution. At this point the commutator reverses the current, and consequently the polarity of the coil, so that there is now repulsion where previously there was attraction, and the coil makes another half-revolution. So the process goes on until the armature attains a very high speed. In general construction there is practically no difference between a dynamo and a motor, but there are differences in detail which adapt each to its own particular work. By making certain alterations in their construction electric motors can be run with alternating current.