Magnetize a sewing-needle and hang it from a thread. Bring the north pole of a bar magnet near the lower end of the needle. If the lower end of the needle happens to be a south pole it will be attracted by the north pole of the bar magnet. If, on the other hand, it is a north pole, it will be repelled and you cannot touch it with the north pole of the bar magnet unless you catch it and hold it.
This fact gives rise to the general law of magnetism: Like poles repel each other and unlike poles attract each other.
Fig. 10.—An Experiment Illustrating that Like Poles Repel Each Other and Unlike Poles Attract.
Another interesting way of illustrating this same law is by making a small boat from cigar-box wood and laying a bar magnet on it. Place the north pole of the bar magnet in the bow of the boat.
Float the boat in a basin of water. Bring the south pole of a second magnet near the stern of the boat and it will sail away to the opposite side of the basin. Present the north pole of the magnet and it will sail back again.
Fig. 11.—A Magnetic Boat.
If the south pole of the magnet is presented to the bow of the boat the little ship will follow the magnet all around the basin.
The repulsion of similar poles may be also illustrated by a number of magnetized sewing-needles fixed in small corks so that they will float in a basin of water with their points down.