Figure 8-9 is a simplified, cutaway drawing of a dc motor. One set of coils, called thearmature coil, goes around with the motor shaft. The other set of coils, called the fieldcoil, is stationary. The current direction is periodically reversed during each rotation bymeans of the commutator. This keeps the force going in the same angular direction, sothe motor continues to rotate rather than oscillating back and forth. The shaft is carriedalong by its own inertia, so that it doesn’t come to a stop during those instants when thecurrent is being switched in polarity.Some dc motors can also be used to generate direct current. These motors containpermanent magnets in place of one of the sets of coils. When the shaft is rotated, a pul-sating direct current appears across the coil.Magnetic data storageMagnetic fields can be used to store data in different forms. Common media for data stor-age include the magnetic tape, the magnetic disk, and magnetic bubble memory.Magnetic tapeRecording tape is the stuff you find in cassette players. It is also sometimes seen onreel-to-reel devices. These days, magnetic tape is used for home entertainment, espe-cially hi-fi music and home video.146 Magnetism8-8At A, pictorial diagram of asimple relay. At B,schematic symbol for thesame relay.