Speedometer, << spee DOM uh tuhr, >> is an instrument that indicates the speed of an automobile or other vehicle. The speedometer display may show speed in miles per hour, kilometers per hour, or both.
There are two types of speedometers: (1) electronic speedometers and (2) mechanical speedometers.
An electronic speedometer consists of a speed-sending unit, a signal conditioner, and an electronic digital or analog readout. The speed-sending unit is connected to the transmission and sends an optical, electric, or magnetic signal to the signal conditioner. In most cases, the signal is a series of pulses that vary in proportion to the vehicle’s speed. The signal conditioner translates the signal and sends it to the electronic readout, which then displays the vehicle’s speed.
A mechanical speedometer indicates speed by means of a dial and a pointer. This type of speedometer is driven by a flexible shaft connected to a set of gears in the vehicle’s transmission. When the vehicle moves, the gears in the transmission turn a core inside the shaft. The core is attached directly to a permanent magnet that lies near a metal cylinder called a speedcup. The revolving magnet sets up a rotating magnetic field that pulls the speedcup and its attached pointer in the same direction that the magnetic field is turning. A hairspring keeps the speedcup steady. The pointer on the speedcup comes to rest where the hairspring balances the force of the revolving magnet. When the vehicle speeds up, the magnet increases its pull on the speedcup, and the speedometer registers a higher speed. When the vehicle stops, the hairspring pulls the pointer back to zero.
A device called an odometer registers the total distance traveled by a vehicle. Many speedometers have a trip odometer that can be reset to zero at the beginning of a trip. Manufacturers typically design mechanical speedometers so that 1,000 revolutions of the flexible shaft will register 1 mile (1.6 kilometers) on the odometer. In electronic speedometers, a specific number of pulses–usually 4,000–equals 1 mile. Thus, electronic odometers convert the number of pulses from the speed-sending unit into the total distance traveled.
