INTRODUCTION TO METRICS

The primary system of measurement used by most of the world is the metric system. The word “metric” is derived from the Greek word “metron,” meaning measure. This system is often identified by the initials SI, which is the abbreviation for Systems Internationale.

STRUCTURE OF THE METRIC SYSTEM

Base Units

throughout the drawing. In such cases, values of less than 57.3° are shown as decimal fractions of a radian. For example, 11° 30' would be shown as 0.20 radian (11° 30' 57.3° = 0.20 rad). Note that one degree approximates

0. 017 radian. This value can be used as a constant for converting a degree value to a radian value and, conversely, to change a radian value to a degree value.

Examples: To change 5° to a radian value, multiply 5° X 0.017 = 0.085 radian.

To change 0.35 radian to degrees, divide 0.35 by 0.017 = 20.6°.

Although not a part of SI, the degree (°), minute ('), and second (") symbols are generally used on draw­ings to represent plane angle dimensions. When an angular measurement includes minutes and seconds, it is preferred practice to show the minutes and seconds as a decimal fraction of a degree. For example, on a metric drawing, 30° 14'35" is best shown as 30.25° or 30.3°.

The steradian is the SI unit of measure for a solid angle. Such an angle has its vertex in the center of a sphere and cuts off an area of the surface of the sphere equal to the area of a square having sides the same length as the radius of the sphere, Figure 22.2.

Solid angle dimensions are not generally shown on two dimensional drawings. Reference is rarely made to steradian as a dimensional value. This unit is more likely to be used in technical data dealing with lighting.

Derived Units

METRIC PREFIXES

In order to specify different values of metric base units of measure a set of prefixes is used, Table 22.4. When a prefix is applied to a base unit, it increases or decreases its value. A prefix that increases the value of a base unit is termed a multiple, and one that decreases its value is termed a submultiple.

The kilogram, for mass (weight), is the only base unit that initially contains a prefix; however, double prefixes are not applied to base units. In this case, it is standard practice to apply a prefix to the gram and not to the kilogram.

Prefixes represent powers of 10 and are com­parable to the decimal numeration system illus­trated in Table 22.4.