Scientists and engineers have mastered the measurement of distance - from fractions of a millimetre to thousands of kilometres. And they can also measure time with the same degree of accuracy. A combination of the two gives us a further important measurement - speed. Speed is the distance travelled by an object in a certain time. Measurements of speed are given by devices named according to the vehicle in which they are found. A ship has a log, an aeroplane has an airspeed indicator. And the most common of all is the speedometer, which is found in motor cars and trains.
An important part of a speedometer is the magnet. A magnet's force is spread out around it in the form of a magnetic field. When a magnet is rotated, its field will rotate with it. The magnet in the speedometer begins to rotate as soon as the vehicle starts to move.
The magnet is driven by a flexible cable which is connected to the front axle of the vehicle. The higher the speed of the vehicle, the higher is the speed of rotation of the magnet. The magnet rotates inside an aluminium ring. The rotating magnetic field brings about ('induces') electric currents within - the aluminium metal. These in turn give rise to a second magnetic field. This magnetic field repels the one produced by the magnet itself.
This force on the ring is called the torque. The faster the magnet turns the higher is the torque. The ring is not free to rotate, but it is able to swing a little. It is held back by a spiral spring. The amount the ring can swing depends on the strength of the torque.
Attached to the ring is a pointer which shows the speed of the vehicle on a suitable scale.
The usual speedometer has a round dial, clearly marked with numbers in tens. Another type has a revolving drum, to which the scale is attached. When the speed increases, the drum turns, and the speed change is shown on the scale through a narrow gap in a panel fixed in front of the scale. The scale may take the form of a coloured band.
Usually, a speedometer is combined with an odometer, which records the distance travelled. The odometer is driven by the same drive shaft or cable that drives the magnet. The movement of the speedometer shaft is transmitted through a system of gears to the distance recorder. This recorder actually counts the number of revolutions made by the vehicle's front wheels. Because a certain number of revolutions are equal to a certain distance travelled by the vehicle, the odometer can be made to give a direct reading of the distance covered.
When the disc that counts the 'units' has made one revolution, the counting disc for the 'tens' is moved on one place. When the 'tens' disc has made one revolution, it then moves the 'hundreds' disc on one place. This process is repeated up to the 'ten thousands' disc.
