and high velocity."

I noticed that the boys stood before this machine in a state of utter bewilderment, bewildered as a man who is told that what he had considered north is really south, bewildered as a man who, having wandered through a maze of city streets, looks up at length and unexpectedly finds the building he has been seeking towering before him. The questions they asked were entirely without thought. "What is inside of it?" "Simply more iron and copper, such as you see on the surface," I replied. "But what makes it go?" "The steam engines, of course, four of which you see, are coupled directly to each dynamo." "But where does it get its electricity?" "Don't forget that you are looking at a generator of electricity. Big mass of iron—rapid motion! That is the whole truth. But it cannot satisfy you as an answer until you have become used to it. We have seen all that we ought to see here to-day. Let us drop the whole matter now, but return to my laboratory to-morrow, and I will give you the next step which will help you."

The boys did no talking upon their return journey. Whether one may say they were thinking or not I cannot tell, but certainly their ideas were incubating.

II

THE DYNAMO, CONTINUED—THE MAGNET

When we had gathered at my laboratory the next day I took down a spool of one pound No. 24 cotton-covered copper wire (Fig. 2 A), which had its centre filled with wire nails. The boys had seen it before and remembered it. With flexible wires I connected the two ends of the wire on this spool to a sensitive ammeter, B, which had its zero in the middle of the scale, and I laid down upon the table a bar magnet, C.

Fig. 2

"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.

"But thanks to Oersted in Denmark in 1819 and Faraday in England in 1821 and Joseph Henry in Albany, N. Y., in 1827, we have learned to make exceedingly powerful magnets by sending a current of electricity in a whirl around the iron. This is the meaning of the coils of copper wire around iron cores in the dynamo, in electric bells, in telegraph sounders, in motors, etc., etc. To prevent the electric current from taking the shortest route, through the iron core or through the successive layers of copper wire, the iron core and the wire must be covered with something like wood or paper or cotton or silk or rubber—such things as electricity does not readily pass through—that is, insulating material.

"Joseph Henry, while teaching in the Albany Academy, was the first to make electro-magnets. There was no such thing as wire covered with an insulating material then in the market, and he wound all his wire with silk ribbon. But in the year 1834 he made magnets which lifted thirty-five hundred pounds, to the astonishment of every one. A pair of such electro-magnets as I have here (Fig. 3), each consisting of one pound of No. 24 cotton covered copper wire, eight hundred feet long, wound in one thousand turns about an iron core two inches in diameter, will lift several hundred pounds: much more than we three can lift, as I shall now show you."

Fig. 3

The cores of the two magnets were bolted fast to an iron beam, and a large bar of iron with a ring in it was laid across the other free ends of the magnet cores. I made connections with the electric lighting circuit (that in my laboratory is what is called a direct current), and sent a current of electricity around the coils. The two boys and I tugged at the ring in the iron bar to no avail. We were unable to pull the iron bar away from the magnet. But when I opened the switch and cut off the electric current, one boy with one finger in the ring lifted the bar with perfect ease.

"Electro-magnets are now made with a magnetic intensity 90,700 times that of the earth's magnetism. Electro-magnets are used for hoisting iron castings weighing many tons. Here is a picture of an electro-magnet lifting a whole wagon load of kegs of nails from the wagon to the hold of a ship.

"Electro-magnets are our only means of utilizing electricity for power. It is the pull of electro-magnets that moves the electric car. Electro-magnets are now used for pulling all the trains out of the Grand Central Depot in New York