speedometer, instrument that indicates speed. A cable from an automotive speedometer is attached to the rear of the transmission of an automobile; the cable turns at a rate proportional to the speed of the car. In a very common type of speedometer the other end of the cable is attached to a simple magnetic device inside the speedometer. In response to the rotating cable, this device moves a needle along a calibrated dial to indicate the speed of the automobile. Another type of automobile speedometer uses centrifugal force and operates in a manner similar to a flyball governor. The airplane speedometer is called an air-speed indicator. A dial that registers the speed in kilometers or miles per hour is actuated by the wind pressure in a tube located where it is not affected by the air stream from the engine. The speed of a watercraft is frequently determined by means of a patent or taffrail log. In this device a small propeller, which is towed astern, revolves as it moves through the water, activating a calibrated dial aboard the vessel. In another type of marine speedometer a tiny fin projects from the hull below the waterline. The resistance of the water to the passage of the fin is converted on a dial into terms of speed in knots.