A gram equals 15.432 grains, a centimeter 0.3937 of an inch. The weight of a body must not be confused with its mass; the mass is the amount of matter contained, while the weight is its specific gravity, which may be expressed by multiplying mass by accelerating effect of gravity, which varies at different parts of the earth’s surface, and may be averaged at 32.2 feet (or 981 centimeters) per second.
The above absolute units are too large or too small for practical use, but the following examples are given:
The dyne (absolute unit of force) is that force which, acting for one second on a mass of one gram, imparts a velocity of one centimeter per second.
The weight of one gram, as above shown, is equal to a force of 1 × 981 = 981 dynes.
The erg, or absolute unit of work is the work requisite to move a body of one centimeter against a force of one dyne.
The weight of one gram is equal to 981 dynes, the work of raising one gram one centimeter, against the force of 981 ergs.
An erg is about equal to 1/13,560,000 of a foot-pound. A foot-pound represents the work required to raise one pound one foot high.
The practical units of most frequent occurrence are: The volt, the ohm and the ampere.
The volt, or measure of E. M. F., or difference of potential, is equal approximately to the E. M. F. possessed by one “Daniell” cell; accurately it is 0.95 of the E. M. F. of a cell.
The ohm, or measure of resistance (sometimes called the British Association or B. A. unit), is approximately equal to the resistance of 129 yards of copper wire ¹⁄₁₆ inch in diameter, or to 106 centimeters of mercury, one square millimeter in section.