660 Table 871 {concluded) 



DEFINITIONS OF UNITS 



MONTH. The anomalistic month = time of revolution of the moon from one perigee to 

 another = 27.55460 days. 

 The nodical month = draconitic month = time of revolution from a node to the same 



node again = 27.21222 days. 

 The sidereal month = the time of revolution referred to the stars = 27.32166 days (mean 

 value), but varies by about three hours on account of the eccentricity of the orbit and 

 " perturbations." 

 The synodic month = the revolution from one new moon to another = 29.5306 days 

 (mean value) = the ordinary month. It varies by about 13 hours. 

 OHM. Unit of electrical resistance. The international ohm is based upon the ohm equal 

 to 10 9 units of resistance of the c.g.s. system of electromagnetic units, and " is repre- 

 sented by the resistance offered to an unvarying electric current by a column of mer- 

 cury, at the temperature of melting ice, 14.4521 grams in mass, of a constant cross 

 section and of the length of 106.3 centimeters " = 10 9 e.m.u. = 1/9 X io" 11 e.s.u. 

 International ohm = 1.01367 B. A. ohms = 1.06292 Siemens' ohms. 

 B. A. ohm = 0.98651 international ohms. 

 Siemens' ohm = 0.94080 international ohms. 

 PARSEC. Distance of star whose parallax is 1". 

 PENTANE CANDLE. Photometric standard. See page 334. 

 PI = ir— ratio of the circumference of a circle to the diameter = 3. 14159265359. 

 POUNDAL. The British unit of force. The force which will in one second impart a 



velocity of one foot per second to a mass of one pound. 

 RADIAN = i8o°/w = 57.29578 = 57 17' 45" = 206265". 

 REAMUR. Thermometric scale. o° C=o° R. 100 C '= 80 ° R. 

 SECOHM. A unit of self-induction = 1 second X I ohm. 

 SLUG. Unit of mass. Mass acquiring acceleration 1 ft./sec. 2 when continuously acted 



upon by 1 lb. wt. 

 SLUG. (Metric) ditto accel. 1 m/sec. 2 , 1 kg weight. 

 TENTH-METER, io" 10 meter = 1 Angstrom. 

 THERM = small calorie == (obsolete). 



THERMAL UNIT, BRITISH = the quantity of heat required to warm one pound of 

 water at its temperature of maximum density one degree Fahrenheit = 252 gram- 

 calories. 

 VOLT. The unit of electromotive force (e.m.f.). The international volt is "the electro- 

 motive force that, steadily applied to a conductor whose resistance is one international 

 ohm, will produce a current of one international ampere. The value of the e.m.f. of 

 the Weston Normal cell is taken as 1.0183 international volts at 20°C=io 8 e.m.u. 

 = 1/300 e.s.u. See page 80. 

 VOLT-AMPERE. Equivalent to Watt/Power factor. 



WATT. The unit of electrical power = io 7 units of power in the c.g.s. system. It is 

 represented sufficiently well for practical use by the work done at the rate of one Joule 

 per second. 

 Watts .= volts X amperes = amperes 2 X ohms = volts 2 /ohms (direct current or alter- 

 nating current with no phase difference). 

 For conversion factors see page 251. 

 Watts X seconds = Joules. 

 WEBER. A name formerly given to the coulomb. 



WORK in ergs = dynes X cm. Kinetic energy in ergs = grams X (cm/sec.) 2 /2. 

 YEAR. See page 601. 

 Anomalistic year = 365 days, 6 hours, 13 minutes, 48 seconds. 

 Sidereal " =365 " 6 " 9 " 9.314 " 



Ordinary " =365 " 5 " 48 "46 + 



Tropical " same as the ordinary year. 



Smithsonian Tables 



