APPENDIX. 

 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 electro-magnetic units, and which 

 is represented 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 o.ooi 11800 of a 

 gram per second." 

 The ampere = i coulomb per second = I volt through I ohm = IO- 1 E. M. U. = 3 X 



10 ' E. S. U.* 



Amperes = volts/ohms = watts/volts = (watts/ohms)*. 

 Amperes X volts = amperes 2 X ohms = watts. 

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

 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 = 29.922 in. = 14.70 Ibs. per sq. in. 

 BAR. A pressure of one dyne per cm. 2 Meteorological " bar "= io tt dynes/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 rrised 



one degree Centigrade at the temperature of maximum density. 

 For conversion factors see page 197. 



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. 



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

 transferred by a current of one international ampere in one second. = icr 1 E. M. U. 

 = 3X io 9 E. S. U. 



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

 CUBIT = 18 inches. 

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



Sidereal day = 86164.10 mean solar seconds. 



DIGIT. 3/4 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 -4- 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. 

 ENERGY. Sec Erg. 

 ERG. C. G. S. unit of work and energy = one dyne acting through one centimeter. 



For conversion factors see page 197. 



FARAD. Unit of electrical capacity. The international farad is the capacity of a con- 

 denser charged to a potential of one international volt by one international coulomb 

 of electricity = io~* E. M. U. = 9 X io u E. S. U. 



The one-millionth part of a farad (microfarad) is more commonly used. 

 Farads = coulombs/volts. 



* E. M. U.=C. G. S. electromagnetic units. E. S. TJ.=C. G. S. electrostatic units. 



