Brian McDaniel, age 13, of Florence, South Carolina, for his question:

What is a micrometer?

You might be able to guess the basic meaning of the world "micrometer" by slicing it exactly in half. The first half is coined from an older word meaning "small" and we find it in such words as "microscope." We meet the second half in "speedometer," where it means "a measuring instrument of speed."

Distance is the measurement from point to point: And the job of doing it never can be perfect. Our remote ancestors paced off distance with strides but modern methods, of course, are more precise. We strive to approach accuracy and with a simple test you can discover, some of the basic problems involved in measuring distances  especially small distances... Use a good ruler and a sharp pencil to fill a plain page of paper with lines, each separat bd by half an inch. It seems simple and the finished job is good enough to serve as a useful page of note paper.

Now test your work. Measure and remeasure the distances between the lines from end, to end. Your sharp eye will detect slight variations With the help of a magnifying glass you will detect variations in the thickness of the lines. These invisible inaccuracies could be disastrous to a machinist designing a delicate instrument or to a researcher work  on a microbe sized problem.

A scientist uses math to solve his equations and math is the clever maneuvering of whole numbers and fractions of whole numbers. But these basic items in his equations, as a rule, are measurements of the basic items in his experiment. This is why scientists are so fussy about measurements. To a bacteriologist, the size of a pencil point is as vague as a stride. He needs super fine instruments to measure super small distances. And one of hid instruments is the micrometer  the measurer of small items.

Actually, there are two types of micrometer. The micrometer caliper uses a screw anti thread. The screw can be turned spirally into a fixed nut and, naturally, this pair of m achine parts is precision made for accuracy and durability. As the screw spirals each turn through the thread of the nut, it advances a certain distance. The distance'between the successive threads is called the pitch. The pitch can be registered on a scale divided into 100 equal parts. A micrometer caliper is small enough to fit into your palm and a micrometers.

The optical micrometer is not so well known and we are likely to find it in a science lab. This super  fine instrument can measure a micron, a metric distance equal to one millionth part of a meter. It uses a small, flat mirror of gem perfect optical glass. The mirror is mounted to reflect the miniscule object, taking advantage of certain known principles of light rays. The miniscule object may be a bacterium with a total length of one 2500th part of an inch. The optical micrometer can be used to measure the dimensions of its length and width to a very fine degree of accuracy.

The caliper micrometer works with the turn of a screw and the optical micrometer worlds with a mirror. Both are fine measurers of small distances but the best of them is not perfect. There are other instruments that strive for better accuracy. One is the electron  microscope, fine enough to show the shadow of an atom. Another is the field ion microscope. But scientists want a still finer instrument capable of viewing the very structure of the atom. Each invention brings us closer to perfect accuracy but by revealing still smaller distances it seems to make this goal hard to reach.