Of course we require the application of very considerable power to urge us to so heavy a task. All the energy which we can get from a few electric batteries might enable us to drive a toy car, but when it comes to turning the wheels of a real car or train, we require a correspondingly greater amount of energy. I may as well tell you quite frankly that we electrons are only the intermediaries or go-betweens. Indeed, you must have noticed that in every case we act merely as a connecting link between matter and the æther, and between the æther and matter. But what I want to tell you of, is the part we play in moving an electric car or railway train. It is really all very simple if you could only see it from our standpoint. Picture a host of us attached to copper atoms in a coil of wire which is being moved through that disturbed æther called a _magnetic field_. We are set in motion immediately. It is true that when we are moved forward into the field we march off in one direction, only to be arrested and made to move off in the opposite direction as we leave the field, but it really makes no difference in our working capabilities as long as we are kept on the move. This is what is actually taking place in the armature of a dynamo as it revolves between the poles of the electro-magnet. There is no peace for us so long as the coil is kept revolving; we are kept in a constant state of rapid to-and-fro motion. [Illustration: A TRAIN IMPELLED BY MOVING ELECTRONS _By permission of Siemens Schuckert Werke_ _Berlin_ It is remarkable that the motion of electrons in an electric conductor can result in the movement of heavy vehicles. How this comes about is explained in Chapter XII.] This is all we electrons do in a dynamo, but when the ends of the outer circuit or mains are brought into contact with the ends of our revolving coil, we set the electrons in the mains surging to and fro in step with ourselves. Man describes this motion of the electrons in the mains as an _alternating electric current_, but by a simple commutator on the dynamo he may arrange that we set the electrons marching in one direction in the mains. This he describes as a _direct electric current_. It is a matter of indifference to us whether man drives our coil round by means of a steam-engine, a water-wheel, or a wind-mill; all that we electrons want is to be kept surging or vibrating to and fro. Now you will be able to appreciate how we electrons get up sufficient motion to enable us to perform what I have described as _our heaviest duties_.