Bert Bagaan, age 16, of Kalispell, Mont., for his question:

WHAT IS A TACHOMETER?

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.  The most common type of tachometer consists of a rubber cone that may be insterted in a hole in the center of a spinning shaft or wheel, a handle to hold it and a speed recording device.

In some cases, this speed recorder resembles the parts of a speedometer. It has a rotating magnet that pulls on an armature restrained by a spring and the movement of the armature gives the rotary speed in revolutions per minute.

In other cases, the rubber cone sets in motion a train of wheels which has a centrifugal arm geared to it. The deflection of this arm under spin shows the number of revolutions per minute.

In still other cases, a train of wheels merely reduces rotations from the rubber cone.

Electric tachometers are commonly used in airplanes. They consist of a voltage generator turned by the engine and connected by electrical wires to an indicator which is a voltmeter. This shows the engine's revolutions per minute on a dial face.