APPENDIX II. 



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 specifica- 

 tions" (5ee pages xxxiv and 251), "deposits silver at the rate of o.ooi 118 of a gramme per 

 second." 

 The ampere = I coulomb per second = 1 volt through i ohm; 

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

 Amperes X volts = amperes ^ X ohms = watts. 



ANGSTROM. Unit of wave-length = io-i» metre. 

 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. 0° C. =29.922 in. = 14.70 lbs. per sq. in. 

 BARAD. C. G. S. unit of pressure = 1 dyne per sq. cm. 

 BOUGIE DECIMALE. Photometric standard; see page 177. 



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

 ature of maximum density, 1° F. = 252 gramme-calories. 

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

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



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

 one degree Centigrade at the temperature of maximum density. 



For conversion factors see page 227. 

 CANDLE. Photometric standard, see page 177. 

 CARAT. The diamond carat =3.168 grains = 0.2053 grammes. 



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

 CARCEL. Photometric standard; see page 177. 

 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. 



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; r/12 the diameter of the sun or moon. 



DYNE. C. G. S. unit of force = that force which acting for one second on one gramme pro- 

 duces a velocity of one centimetre per second. 



= weight in grammes divided by the acceleration of gravity in cm. per sec. 

 ENERGY. See Erg. 

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



For conversion factors see page 227. 

 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. 



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



Farads = coulombs/ volts. 

 FOOT-POUND. The work which will raise one pound one foot high. 



For conversion factors see page 227. 

 FOOT-POUNDALS. The English unit of work = foot-pounds /g. 



For conversion factors see page 227. 

 g. The acceleration produced by gravity. 



GAUSS. A unit of intensity of magnetic field = lO^ C. G. S. units. 

 GRAMME. See page 6. 



