It has been a source of amusement to me to see people perfectly mystified by the fact that they can get no electric current unless they have a complete circuit. What else could they expect? How could man march if he had no road to march on? You see, the reason for our march is that we wish to escape from the overcrowding on the zinc, and we are forced towards the copper. The atoms composing the wire are our stepping-stones, and if there is not a complete chain of atoms we are helpless. You have already heard how we can jump an air-space under very great pressure, but that condition does not exist in the present case. When we are disturbed by the chemical action of the battery, we should prefer to have a short-cut from the zinc to the copper, but if the only path man gives us is by way of a long wire, then we must be content to travel that road, in order to reach the copper. It is a matter of little moment to us what arrangement man makes as long as he gives us a complete path. For instance, he may lead us out from the zinc to a distant telegraph instrument, and then, instead of providing a second wire to take us back to the battery, he may conduct us by a short wire to the earth. We are quite content to lose ourselves in this great reservoir, provided man places another short wire from the earth to the copper of the battery at the other end of the line. Then as we slip off at the one end of the line, an equal number of electrons can climb up at the other end, and thus enable all our friends in the long wire to keep up a steady march.
This march of ours is not merely a means of transporting ourselves from one place to another; it is to enable us to do work. It is only when we are in motion that we can do useful work, for we must move before we can disturb the æther, and it is by means of the æther that we transmit energy.
If you place a magnetic needle or mariner's compass near a wire along which we are making a steady march, you will find that we can affect our fellow-electrons who are stationed within the magnetic needle. We cause the needle to swing round and take up a position at right angles to our line of march. We succeed in doing this because these electrons in the magnetic needle are on the move also. But this reminds me that I have never told you how we produce that æther disturbance which you call magnetism.
When, as children, you played with toy magnets in the nursery, little did you think that there was a host of tiny electrons amusing you. And yet we electrons are responsible entirely for all magnetic effects, as I shall proceed to explain.