Kelvin temperature scale. — Scale of temperature based on equal work for 

 equal temperatures for a working substance in a carnot cycle = Celsius (Centi- 

 grade) scale + 273.16. 



Langley (ly). — A new unit of radiation, surface density, bas been sug- 

 gested ~ which equals 1 calorie (15°C) per cm 2 . 



Latent heat. — Quantity of beat required to change the state of a unit mass 

 of matter. 



Pyron. — A unit of radiant intensity = 1 cal cm" 2 min" 1 . 



Radiant energy. — Energy traveling in the form of electromagnetic waves. 



Radiant temperature. — The temperature obtained by use of a total radia- 

 tion pyrometer when sighted upon a non-blackbody. This is always less than 

 the true temperature. 



Rankin temperature scale. — Absolute Fahrenheit scale = Fahrenheit 

 scale + 459.7. 



Reaumur temperature scale. — A scale based upon the freezing point of 

 water taken as 0°R and the boiling point of water taken as 80°R. 



Specific heat. — Ratio of the heat capacity of a substance to the heat capacity 

 of an equal mass of water. When so expressed, the specific heat is a dimen- 

 sionless number. 



Standard temperature. — A temperature that depends upon some char- 

 acteristic of some substance, such as the melting, boiling, or freezing point, that 

 is used as a reference standard of temperature. 



Thermal capacitance.— The heat capacity of a body is the limiting value, 



. &Q 

 as T approaches zero, of the ratio —=:< where AT is the rise in temperature 



resulting from the addition to the body of a quantity of heat equal to AQ. 



Thermal conductivity. — Quantity of heat, Q, which flows normally across 

 a surface of unit area per unit of time and per unit of temperature gradient 

 normal to the surface. In thermal units it has the dimensional formula 

 (Hd^L^T' 1 ) or {ML- l T l ), in mechanical units (MLT^ 1 ). 



Thermodynamic temperature. — See Kelvin temperature scale. 



Thermodynamics. — Study of the flow of heat. 



Thermodynamic laws: Zcroth laiv. — Two systems that are in thermal 

 equilibrium with a third are in thermal equilibrium with each other. First law: 

 When equal quantities of mechanical effect are produced by any means what- 

 ever from purely thermal effects, equal quantities of heat are put out of 

 existence or are created. Second lazv: It is impossible to transfer heat from 

 a cold body to a hot body without the performance of mechanical work. Third 

 lazv: It is impossible by any means whatever to superpose only the images of 

 several light sources to obtain an image brighter than the brightest of the 

 source. 



7 Aldrich et al., Science, vol. 106, p. 225, 1947. 



SMITHSONIAN PHYSICAL TABLES 



