Page:Handbook of Meteorology.djvu/265



The construction and evolution of the system of electrical units now in general use began in Europe where the metric system originated. Two of the basic units, the centimeter and the gram, are metric units; the second of time is universally employed in time measurements.

The meter, 3.2808 feet = 39.37 inches, is theoretically the one ten-millionth part of the earth’s quadrant; actually it is the length of a metal rod differing from the theoretical value by about three-fourths of a millimeter. The prefixed multiples, deka-, hecto-, kilo- and mega- are in ten-fold ratio. They indicate respectively 10, 100, 1000 and 1,000,000. The prefixed decimal divisions, deci-, centi-, milli- and micro- indicate tenths, hundredths, thousandths and millionths.

The C. G. S. units are generally used in the United States in chemical and in abstract physical determinations. On account of their inconvenient magnitudes their use has not extended to mechanics; and for this reason the employment of them is strenuously resisted by manufacturers in every line.

Units of the C. G. S. System. — The centimeter, the one-hundredth part of the meter, is the unit of length. 1 cm = 0.0328 foot.

The gram, the unit of mass, is the weight of 1 cu cm of pure water under standard conditions. 1 g = 0.0022 pound = 15.432 grains. The weight of a cubic centimeter of a substance also represents its specific gravity.

The square centimeter is the unit of area. 1 sq cm or cm2 = 0.001076 sq ft = 0.1 550 sq in.