658 Table 871 



DEFINITIONS OF UNITS 



ACTIVITY. Power or rate of doing work; unit, the watt. • 



AMPERE. Unit of electrical current. The international ampere, " which is one-tenth of 

 the unit of current of the c.g.s. system of electromagnetic units, and which is repre- 

 sented sufficiently well for practical use by the unvarying current which, when passed 

 through a solution of nitrate of silver in water, and in accordance with accompanying 

 specifications, deposits silver at the rate of 0.00111800 of a gram per second." 

 The ampere = 1 coulomb per second = 1 volt through 1 ohm = io" 1 e.m.u. = 3 X io' J 



e.s.u. 

 Amperes = volts/ohms = watts/volts = ( watts/ohms )i 

 Amperes X volts = amperes 2 X ohms = watts. 

 ANGSTROM. Unit of wave length = io" 10 meter. 



ASTRONOMICAL UNIT. Mean distance earth to sun, 149,500,000 km. 

 ATMOSPHERE. Unit of pressure. 

 English normal = 14.7 pounds per sq. in. = 29.929 in. = 760.18 mm Hg. 32 F. 

 French " — 760 mm of Hg. o°C = 29.922 in. = 14.70 lbs. per sq. in. 



AVOGADRO NUMBER. Number of molecules per mole 6.064 X io 23 mole" 1 . 

 BAR. International unit of pressure io 6 dyne/cm 2 , g = 980.616 cm/sec 2 . 

 BARYE. c.g.s. pressure unit, one dyne/cm 2 . 



BRITISH THERMAL UNIT. Heat required to raise one pound of water at its tem- 

 perature of maximum density, i° F. = 252 gram-calories. 

 CALORIE. Small calorie = gram-calorie = therm = quantity of heat required to raise 

 one gram of water at its maximum density, one degree Centigrade. 

 Large calorie = kilogram-calorie = 1000 small calories = one kilogram of water raised 



one degree Centigrade at the temperature of maximum density. 

 For conversion factors sec page 251. 

 CANDLE, INTERNATIONAL. The international unit of candlepower maintained 

 jointly by national laboratories of England, France and United States of America. 

 CARAT. The diamond carat standard in U. S. = 200 milligrams. Old standard = 205.3 

 milligrams = 3.168 grains. 

 The gold carat : pure gold is 24 carats ; a carat is 1/24 part. 

 CIRCULAR AREA. The square of the diameter := 1.2733 X true area. 



True area = 0.785398 X circular area. 

 CIRCULAR INCH. Area of circle one inch in diameter. 



COULOMB. Unit of quantity. The international coulomb is the quantity of electricity 

 transferred by a current of one international ampere in one second = io" 1 e.m.u. 

 = 3Xio 9 e.s.u. 

 Coulombs = (volts-seconds) /ohms = amperes X seconds. 

 CUBIT = 18 inches. 



DALTON. Unit of mass, 1/16 mass of oxygen atom. 1.65 X icf 2 * g. 

 DAY. Mean solar day = 1440 minutes = 86400 seconds = 1.0027379 sidereal day. 



Sidereal day = 86164.10 mean solar seconds. 

 DIGIT. £ inch; 1/12 the apparent diameter of the sun or moon. 

 DIOPTER. Unit of "power" of a lens. The number of diopters = the reciprocal of the 



focal length in meters. 

 DYNE, c.g.s. unit of force = that force which acting for one second on one gram produces 

 a velocity of one cm per sec. = ig -j- gravity acceleration in cm/sec. /sec. 

 Dynes = wt. in g X acceleration of gravity in cm/sec. /sec. 

 ELECTROCHEMICAL EQUIVALENT is the ratio of the mass in grams deposited in 

 an electrolytic cell by an electrical current to the quantity of electricity. 



Smithsonian Tables 



