Units, in scientific language, are the arbitrarily chosen standards in terms of which different quantities are expressed. The idea is familiar in common life. No commercial transaction can be carried out without a clear understanding as to the units employed. The pound, the yard, the mile, the acre, the gallon, the hour, the dollar, and so on, are examples of ordinary units, which have become more and more definitely fixed as civilisation advanced. In these days of scientific exactitude great care must be taken in fixing the units of commerce and in determining the ratios of the units used in different countries for the same commodities. Thus the English pound has a definite relation to the French kilogramme, and the yard to the metre. The relations between money units vary, however, for the reason that the value of money depends on fluctuating commercial conditions. See WEIGHTS AND MEASURES, MONEY, EXCHANGE, BIMETALLISM, CURRENCY.
In science we distinguish fundamental and derived units. The fundamental units are so named because in terms of them all other physical units can be expressed. It has been found convenient to take the units of length, mass, and time as the fundamental units; and the centimetre, gramme, and second have been accepted in this sense by the whole scientific world. As thoroughly scientific a system can of course be based on any other chosen units, such as the foot, pound, and second. From these fundamental units all others are derived by definition. Thus the scientific unit of surface is the square, and the unit of volume the cube, whose side is the unit of length. The English acre and gallon, which have no simple relation to the inch, foot, yard, or mile, are essentially unscientific. Other scientific units, such as those of velocity, force, work, involve in their definitions two or all of the fundamental units. Then a growingly important set of derived units are those of electrical and magnetic quantities, such as the ampere, the ohm, the volt, the watt, and so on. In the accepted scientific system, called for brevity the C.G.S. (Centimetre, Gramme, Second) system, one great merit is its purely decimal character. By an extension of the French method of prefixes (centi, deci, kilo, &c.) we are supplied with an unlimited stock of unit-names. Thus the microfarad is an electric capacity which is one-millionth of the farad, and the megadyne a force equal to one million times the dyne or C.G.S. unit of force. Without such prefixes we should have to use at times either very large or very small numbers; they serve, indeed, the same function as a change from inches to miles, or tons to pounds. A general discussion of the significance of units is given in all our best modern text-books on the various departments of physics. See also Everett's Units and Physical Constants (3d ed. 1891); and see ELECTRICITY, FORCE, WATT.