In actual practice messages are not ordinarily sent long distances over a direct line, but are automatically transferred to new lines at definite points. For example, a message from New York to Chicago does not travel along an uninterrupted path, but is automatically transferred at some point, such as Lancaster, to a second line which carries it on to Pittsburgh, where it is again transferred to a third line which takes it farther on to its destination. CHAPTER XXXIII MAGNETS AND CURRENTS 304. In the twelfth century, there was introduced into Europe from China a simple instrument which changed journeying on the sea from uncertain wandering to a definite, safe voyage. This instrument was the compass (Fig. 221), and because of the property of the compass needle (a magnet) to point unerringly north and south, sailors were able to determine directions on the sea and to steer for the desired point. [Illustration: FIG. 221.--The compass.] Since an electric current is practically equivalent to a magnet (Section 296), it becomes necessary to know the most important facts relative to magnets, facts simple in themselves but of far-reaching value and consequences in electricity. Without a knowledge of the magnetic characteristics of currents, the construction of the motor would have been impossible, and trolley cars, electric fans, motor boats, and other equally well-known electrical contrivances would be unknown. 305. The Attractive Power of a Magnet. The magnet best known to us all is the compass needle, but for convenience we will use a magnetic needle in the shape of a bar larger and stronger than that employed in the compass. If we lay such a magnet on a pile of iron filings, it will be found on lifting the magnet that the filings cling to the ends in tufts, but leave it almost bare in the center (Fig. 222). The points of attraction at the two ends are called the poles of the magnet. [Illustration: FIG. 222.--A magnet.] If a delicately made magnet is suspended as in Figure 223, and is allowed to swing freely, it will always assume a definite north and south position. The pole which points north when the needle is suspended is called the north pole and is marked _N_, while the pole which points south when the needle is suspended is called the south pole and is marked _S_. A freely suspended magnet points nearly north and south. A magnet has two main points of attraction called respectively the north and south poles. [Illustration: FIG. 223.--The magnetic needle.]