Parts of a Speedometerby Joshua Smyth
Speedometers keep people alive. While driving, it is easy to lose track of how fast the car is going. A quick check of the speedometer tells you whether you are traveling at a safe or legal speed for the road and weather conditions. They emerged on the market in 1910, but took a while to become standard equipment. Early speedometers were mechanical; electronic ones only hit the market in the 1990s.
Mechanical speedometers connect to the transmission of the car, not the wheels. They do so by a drive cable, which is a collection of small springs wound very tightly around a central wire (also known as a mandrel). This construction makes the cable flexible enough to be bent and snaked through the body of the car to the instrument panel. The cable is connected to a set of gears attached to the transmission, which is carrying rotation from the engine to the wheels. When the transmission turns, it turns the gears, which turn the mandrel wire in the drive cable. This turning is transmitted along the mandrel wire to the instrument itself.
The drive cable runs from the transmission to the instrument, where it is connected by a spiral gear to a permanent magnet.
The magnet connected by a drive cable to the transmission sits in a metal piece shaped like a cup. This piece is attached to the needle that the driver sees, so that when the transmission turns the wheels, the motion is transmitted through the drive wire to the magnet. The magnet spinning in the cup creates a rotating magnetic field, which creates small electrical eddy currents in the speedcup. This exerts a small bit of torque on the speedcup, pushing it to turn in (and the needle) the direction that the magnetic field is turning. The faster the transmission turns, the stronger the magnetic field pushing the speedcup, and the farther the needle will turn.
A hairspring resists the force of the speedcup enough to hold the needle at zero when the car isn't moving. This ensures that the needle reading reflects the car's real speed.
Connected to the speedcup, the needle points out the car's speed on the instrument dial in the car's cabin.
Electronic speedometers use a metal disk with tiny teeth, mounted on the crankshaft or transmission and surrounded by a round magnet. The teeth spin and cause pulsations in the magnetic field, which are transmitted to a small computer that counts them and translates the number into turns of the crankshaft and wheels, and thus the speed of the car. They may still have a needle and dial, but these are no longer required for the system to function.
- speedometer image by Larry from Fotolia.com