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 specifi- 

 cations" {see pages xxxvi, 261), "deposits silver at the rate of 0.001118 of a gram per 

 second. 

 The ampere = i coulomb per second = i volt through i ohm = io~i E. M. U. = 3 X 



io9 E. S. U.* . / u nJ 



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

 Amperes X volts = amperes ^ X ohms = watts. 

 ANGSTROM. Unit of wave-length = lo-i" meter. 

 ATMOSPHERE. Unit of pressure. ^ 



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

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

 BOUGIE DECIMALE. Photometric standard; see page 178. 



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

 ature of maximum density, i " F. = 252 gram-calories. 

 CALORY. Small calory = gram-calory = therm = quantity of heat required to raise one 

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

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



one degree Centigrade at- the temperature of maximum density. 

 For conversion factors see page 237. 

 CANDLE. Photometric standard, see page 178. 



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. 

 CARCEL. Photometric standard; see page 178. 

 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. = I0~^ E. M. U. 

 = 3 X 108 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 pro- 

 duces a velocity of one centimeter per second. 

 = weight in grams divided by the acceleration of gravity in cm. per 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. See Erg. 

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



For conversion factors see page 237. 

 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. = lo-s E. M. U. = 9 X 10" 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. U. = C. G. S. electrostatic units. 



