Current-carrying loop and the Lorentz force

Ampère forces act on the opposite sides of a current-carrying loop in a magnetic field; by the left-hand rule they point in opposite directions, so they turn the loop. The loop turns and stops when its plane is perpendicular to the field lines. The turning effect is directly proportional to the current. This phenomenon is the basis of moving-coil (magnetoelectric) measuring instruments (ammeter, voltmeter): a coil and a pointer sit between the poles of a strong permanent magnet, and a spiral spring holds the coil. When current flows the coil turns and twists the spring; the spring resists the turn, so the pointer deflects through an angle proportional to the current. An ammeter is connected in series; mind the poles and do not pass more current than the measuring limit.