Anthony DaSilva, age 12, of Jackson, Miss., for his question:
HOW DOES A SPEEDOMETER WORK?
A speedometer is an instrument that reports the speed of an automobile or other vehicle. The instrument is powered by a flexible shaft that connects to a set of gears in the transmission of the vehicle. Some speedometers run from a small electric generator.
The gears in the transmission take into account the tire size and the axle ratio. When the venicle moves, the gears turn a core inside the shaft. The core is attached directly to a permanent magnet that lies near 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 stops, the hairspring pulls the pointer to zero. When the vehicle speeds up, the magnet increases its pull on the speedcup. This causes the speedometer to register a higher speed.
The dial of a speedometer has numbers and a pointer to indicate to the driver how fast the vehicle is traveling. The speed is shown in miles per hour in American built automobiles. In many of the cars built in other countries, the speed is indicated in kilometers per hour.
A device called an odometer registers the total distance traveled by a vehicle. Also, many speedometers have a trip odometer that can be reset to zero at the beginning of a trip.
Manufacturers design speedometers so that 1,000 revolutions of the flexible shaft will register one mile on the odometer, and 1,000 revolutions a minute will indicate a speed of 60 miles an hour.
Speedometers require little care. But at times dirt and excess grease will make them inaccurate. The experts say that they should be lubricated about every 10,000 miles.
An instrument in the same classification as the odometer is the pedometer. This is a small instrument that measures the distance a person walks.
The pedometer, which looks like a watch, is usually carried in the pocket. With each step, the motion of the body causes a small lever to move. This lever records the number of steps taken. To find out how far he has welked, the walker must find the average length of his step and multiply it by the number of steps recorded.
In some pedometers, a mechanism accounts for the length of a step and actually measures the distance walked.
A tachometer is a .device that is used for measuring the speed of rotation of a spinning shaft or wheel, usually in terms of revolutions per minute.