A speedometer is a vehicle instrument that measures the instantaneous speed. The most common form of speedometer relies on the interaction of a small permanent magnet affixed to the rotating cable with a small aluminum cup affixed to the shaft of the pointer. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet drags the cup--and thus the speedometer pointer--in the direction of its rotation with no mechanical connection between them. The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.