[demo] shows a rectangular coil mounted on delicate pivot bearings between the poles of a permanent magnet. The cylindrical soft iron core and the curved pole faces shape the magnetic field so that B is uniform and directed radially. As the coil rotates, its plane remains parallel to B, keeping sin= 1, in the equation = BNIAsin. The coil is restrained by a hairspring, which is usually used as one conductor carrying current to the coil. When current is passed through the coil, it experiences a torque which causes it to turn. The coil turns until the torque due to the interaction of current and magnetic field is balanced by the opposing torque exerted by the hairspring. Since the angle of deflection of the spring is proportional to the torque, by Hooke's Law, the angular deflection of the pointer attached to the coil is a measure of the current in the coil. By connecting the coil in series with a current balance, we can calibrate the needle positions in amperes. In very brief outline, we have described the principle and construction of an analog ammeter. We refer to this device subsequently in both problems and lecture. Ammeters are made in a wide variety of sizes and sensitivities. Delicate and sensitive instruments will have small, light coils, sometimes mounted, like the balance wheel of a watch, on jewel bearings.

Another way of utilizing the torque on a current-carrying coil is in the construction of a motor.

In a motor, a strong magnetic field is usually supplied by an electromagnet, the current for which comes from the source driving the motor. A rugged coil is mounted on a shaft which is free to turn in a set of bearings. When current is passed through the coil, it rotates to an orientation in which the torque becomes zero. If, because of its inertia, it continues past this point with no alteration in the current, it experiences a torque in the opposite direction and returns to the equilibrium position. If, however, we arrange to reverse the battery connections to the coil just as it passes through the equilibrium position, the direction of the the current in the coil is reversed and the torque continues acting in the original direction. Under these circumstances, the coil can rotate continuously. With large currents in the coil and a strong magnetic field supplied by the electromagnet, a large torque will be available at the rotating shaft. By suitable connections of gears, belts, or pulleys, this torque can be utilized to run machinery and do other kinds of mechanical work.

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