[Illustration: Fig. 1.—The Card of a Mariner’s Compass, Showing the "Points."] In the center of the card is a conical socket poised on an upright pin fixed in the bottom of the bowl, so that the card hanging on the pin turns freely around its center. On shipboard, the compass is so placed that a black mark, called the _lubber’s line_, is fixed in a position parallel to the keel. The point on the compass-card which is directly against this line indicates the direction of the ship’s head. Experiments with Magnetism The phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play an important part in the construction of almost all electrical apparatus. Dynamos, motors, telegraphs, telephones, wireless apparatus, voltmeters, ammeters, and so on through a practically endless list, depend upon magnetism for their operation. *Artificial Magnets* are those made from steel by the application of a lodestone or some other magnetizing force. The principal forms are the Bar and Horseshoe, so called from their shape. The process of making such a magnet is called _Magnetization_. [Illustration: Fig. 2.—A Bar Magnet] Small horseshoe and bar magnets can be purchased at toy-stores. They can be used to perform very interesting and instructive experiments. [Illustration: Fig. 3.—A Horseshoe Magnet] Stroke a large darning-needle from end to end, always in the same direction, with one end of a bar magnet. Then dip the needle in some iron filings and it will be found that the filings will cling to the needle. The needle has become a magnet. Dip the bar magnet in some iron filings and it will be noticed that the filings cling to the magnet in irregular tufts near the ends, with few if any near the middle. [Illustration: Fig. 4.—A Magnetized Needle and a Bar Magnet which have been dipped in Iron Filings.] This experiment shows that the attractive power of a magnet exists in _two opposite_ places. These are called the poles. There exists between magnets and bits of iron and steel a peculiar unseen force which can exert itself across space. The power with which a magnet attracts or repels another magnet or attracts bits of iron and steel is called *Magnetic Force.* The force exerted by a magnet upon a bit of iron is not the same at all distances. The force is stronger when the magnet is near the iron and weaker when it is farther away. [Illustration: Fig. 5.—The Lifting Power of a Bar Magnet. _It must be brought closer to the nails than the tacks because they are heavier_.] Place some small carpet-tacks on a piece of paper and hold a magnet above them. Gradually lower the magnet until the tacks jump up to meet it.