3IO APPENDIX. 



GRAMME-CENTIMETRE. The gravitation unit of work=g. ergs. 

 For further conversion factors see page 227. 



HEAT UNIT. See Calory. 



HEAT OF THE ELECTRIC CURRENT generated in a metallic circuit without self- 

 induction is proportional to the quantity of electricity which has passed in coulombs 

 multiplied by the fall of potential in volts, or is equal to (coulombs X volts) 74.181 in 

 small calories. 



The heat in small or gramme-calories per second = (amperes 2 X ohms) /4.i8i= volts 2 / 

 (ohms X 4.181) = (volts X amperes) /4.i8i = watts/4.i8i. 



HEAT. Absolute zero of heat = -273 Centigrade, -4594 Fahrenheit, -218.4 Reaumur. 



HEFNER UNIT. Photometric standard; see page 177. 



HENRY. Unit of induction. It is "the induction in a circuit when the electromotive force 

 induced in this circuit is one international volt, while the inducing current varies at 

 the rate of one ampere per second." 



HORSE-POWER. The practical unit of power = 33,000 pounds raised one foot x per min- 

 ute. 



JOULE. Unit of work = io 7 ergs. 



Joules = (volts 2 X seconds) /ohms = watts X seconds = amperes 1 X ohms X sec. 

 For conversion factors see page 227. 



JOULE'S EQUIVALENT. The mechanical equivalent of heat = 4.18 1 XIO T ergs. See 

 page 227. 



KILODYNE. looo dynes. About I gramme. 



LITRE. See page 6. 



MEGABAR. Unit of pressure =0.987 atmospheres. 



MEGADYNE. One million dynes. About one kilogramme. 



METRE. See page 6. 



METRE CANDLE. The intensity lumination due to standard candle distant one metre. 



METRET. An exponential subdivision of the metre. The ordinal number before the word 

 metre denotes the power of ten serving as the divisor; e. g., a tenth-metret = io- 10 

 i /io 10 metre. The first metret is the decimetre, the second, the centimetre, etc. 



MHO. The unit of electrical conductivity. It is the reciprocal of the ohm. 



MICRO. A prefix indicating the millionth part. 



MICROFARAD. One millionth of a farad, the ordinary measure of electrostatic capacity. 



MICRON. (M) =one millionth of a metre. 



MIL. One thousandth of an inch. 



MILE. See pages 5, 6. 



MILE, NAUTICAL or GEOGRAPHICAL = 6080.204 feet. 



MILLI-. A prefix denoting the thousandth part. 



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. 2 1222 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 io 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 grammes in mass, of a constant cross 

 section and of the length of 106.3 centimetres." 

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

 B. A. ohm =0.98651 international ohms. 

 Siemens' ohm =0.94080 international ohms. See page 261. 



PENTANE CANDLE. Photometric standard. See page 177. 



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 veloc- 

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



RADIAN = 180 /* = 57-29578 = 57 i?' 45" =206625". 



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



THERM = small calory = quantity of heat required to warm one gramme of water at its 

 temperature of maximum density one degree Centigrade. 



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

 at its temperature of maximum density one degree Fahrenheit =252 gramme-calories, 



VOLT. The unit of electromotive force (E. M. F.). The international volt is "the 

 electromotive force that, steadily applied to a conductor whose resistance is one inter- 

 national ohm, will produce a current of one international ampere, and which is repre- 

 sented sufficiently well for practical use by 1000/1434 of the electromotive force be- 



