Dalton (atomic mass unit M ). — Unit of mass, 1/16 mass of oxygen ( 8 13 ) 

 atom, 1.66080 x 10 24 g (Phys. scale). (See Table 26.) 



Density. — The mass per unit volume. The specific gravity of a body is the 

 ratio of a density to the density of a standard substance. Water and air are 

 commonly used as the standard substance. 



Digit. — 3/4 in.; 1/12 the apparent diameter of the sun or moon. 



Diopter. — Unit of "power of a lens." The diopter = the reciprocal of the 

 focal length in meters. 



Dyne. — The cgs, unit of force = that unbalanced force which acting for 

 1 second on body of 1 gram mass produces a velocity change of 1 cm/sec. 



Energy. — The work done by a force produces either a change in the velocity 

 of a body or a change of its shape or position or both. In the first case it pro- 

 duces a change of kinetic energy, in the second, of potential energy. 



Erg. — The cgs unit of work and energy = the work done by 1 dyne acting 

 through 1 centimeter. 



Fluidity. — Reciprocal of viscosity. 



Foot-pound. — The work which will raise 1 pound, body 1 foot high for 

 standard g. 



Foot-poundal. — The work done when a force of 1 poundal acts through 

 1 foot. 



Force (/). — Force is the agent that changes the motion of bodies and is 

 measured by the rate of change of momentum it produces on a free body. 



Gal = gravity standard = an acceleration of 1 cm see 2 . 



Giga = 10 9 . 



Gram. — The standard of mass in the metric system. (See Table 31.) 



Gram-centimeter. — The cgs gravitation unit of work. 



Gram-molecule. — The mass in grams of a substance numerically equal to 

 its molecular weight. 



Gravitation constant. — (G, in formula F = Gm 1 m 2 /r 2 ) = 6.670 X 10" 8 dyne 

 cm 2 g" 2 . 



Gravity (g). — The attraction of the earth for any mass. It is measured by 

 the acceleration produced on the mass under standard conditions. This ac- 

 celeration g equals 980.665 cm sec" 2 or 32.17 ft sec" 2 . 



Horsepower. — A unit of mechanical power. The English and American 

 horsepower is defined by some authorities as 550 foot-pounds/sec and by 

 others as 746 watts. The continental horsepower is defined by some authori- 

 ties as 75 kgm/sec and by others as 736 watts. 



Joule. — Unit of work (energy) = 10 7 ergs. Joules = (volts 2 x sec)/ 

 ohms = watts X sec = amperes 2 x ohms X sec = volts X amperes X sec. 



Kilodyne. — 1,000 dynes. About 0.980 gram weight. 



SMITHSONIAN PHYSICAL TABLES 



