How Does the Speedometer of a Motorcar Work?
We can get an idea of how a speedometer works if we imagine that we hold in our hand a small wheel with a spindle through it, and a fine coil spring attached to the spindle. If we keep tapping the edge of the wheel, it will go round and gradually wind the spring up, but as soon as we stop tapping, the pull of the spring will send the wheel back to its original position. If a hand were fitted to the spindle so as to move over a properly numbered disc, a measure of speed would be obtained. Thinking of this, we can look at two popular types of the speedometer, the rotary and the dial types.
From the driving shaft of an automobile a flexible tube runs to the dial box. When the car is moving a flexible cable within the tube is revolved by means of worm gears attached to the driving shaft. At the top of the flexible cable is a circular magnet, and this therefore revolves. Fitting over the magnet, but not touching it, is (in the rotary type) an inverted drum on which are printed figures to show the car’s speed. The rotating magnet acts with magnetic force on the drum, and this magnetic influence has the same effect as the finger tapping on the wheel: it causes the drum to revolve. The faster the magnet revolves—that is, the faster the car is moving - the greater the magnetic pull on the drum, and a number is brought to the opening showing the speed of the car.
In the dial type the magnet fits into the centre of the indicator body, but is not in contact with it. As the magnet revolves, a current moves a pointer round a fixed dial. Movement of the drum or pointer is restrained by the counter-tension of a coiled spring. As the magnetic attraction increases with the speed, the resistance of the spring is gradually overcome, and the hand or drum moves round. As the car slows and stops, the magnetic force declines too, and the hand or drum goes back to zero.
The same driving power carried up the flexible tube actuates gears which turn the mileage recorder. This part of the speedometer which registers mileage has its own special name. It is called the odometer (from two Greek words meaning way and measure). One type measures separately the total mileage and the trip mileage. The trip mileage is registered up to ioo or iooo miles. In this recorder the extreme right-hand figure indicates a fraction of a mile. When the right - hand figure has recorded 9, and o is appearing, the disc on which the o is engages the disc immediately to the left which thus advances one figure higher, and so on right across. When 99.9 or 999.9 miles are recorded and the final one-tenth of a mile is just completed, the recorder shows zero in all places. At the start of a day’s run the trip recorder can be set at zero by turning a knob which advances the figures.
The sum of all these trip mileages is shown on the total mileage recorder. When the total mileage has reached 99,999 miles the addition of another mile sets all these numbers back to zero again.
