When a current of electricity is sent through a dynamo machine in the same direction as the current flows when the dynamo is being used as a generator, then the field magnets are polarised in their normal manner, and the magnetism of the armature is such as to cause it to rotate in the opposite direction to that in which it turns when generating a current. The dynamo now acts as a motor. If now the current be sent through the dynamo in the opposite direction, the field magnets are polarised in the opposite manner, i.e., a north pole is found where a south pole was before. The same is true of the magnetism of the armature.