"Here," I said, "is a dynamo complete." The bar magnet furnishes the 'field' and this spool of copper wire, _A_, which I will move back and forth immediately over the magnet from end to end, is 'the armature.' _D_ and _e_ are the line wires and the circuit is completed through the ammeter to show whether we are generating electricity. And now as I move this armature along the field you see the needle of the ammeter move to the right from zero to ten. When the armature is moved in the opposite direction along the field the needle moves in the opposite direction past zero and on to ten at the left. The moving of the needle in the ammeter shows that we are generating electricity. The swinging to and fro of the needle shows that we are generating an alternating current of electricity. It is a mere matter of detail whether we move the armature or the field, as I will show you by letting the spool A rest quietly upon the table and moving the magnet to and fro lengthwise across the end of the spool. Or I may accomplish the same results by moving them both in opposite directions. It is simply necessary that they move _with reference to each other_. Some dynamos are made with stationary fields and rotating armatures, some with stationary armature and rotating fields, and some with both parts designed to rotate in opposite directions. "Magnetism is not confined to the magnet. It extends more or less widely into the region about it. It is this region affected by the magnet that we designate its magnetic field. By bringing this sensitive compass needle into the region of this bar magnet from all directions, I show you that it has a slight power to change the direction of the needle when about a foot away. This power grows rapidly greater as the distance grows less. Of course its field extends rather indefinitely, but we may say that this particular magnet has an appreciable field extending about one foot in all directions from it. We find upon examination that some magnets have bigger and stronger fields than others, that all have their strongest fields when first magnetized and lose their strength gradually, _but never entirely_. We find that hardened iron and steel hold magnetism longer than soft iron, _but all iron is magnetized somewhat at all times_. Iron that is feebly magnetized can be made into a strong magnet by bringing it into a strong magnetic field. The earth is a feeble magnet, and that is why it gives direction to the compass needle. That is also probably the reason why every piece of iron upon the earth is a magnet, or, to put the cause back another step, we may say that whatever causes the earth to be a magnet also causes every piece of iron upon the earth to be likewise a magnet.