8 



There are various calories depending upon the interval chosen. Sometimes 

 the unit is written as the gram-calorie or the kilogram-calorie, the meaning of 

 which is evident. There is some tendency to define the calorie in terms of its 

 mechanical equivalent. Thus the National Bureau of Standards defines the 

 calorie as 4.18400 joules. At the International Steam Table Conference held 

 in London in 1929 the international calorie was defined as 1/860 of the inter- 

 national watt hour (see Table 7), which made it equal to 4.1860 international 

 joules. With the adoption of the absolute system of electrical units, this be- 

 comes 1/859.858 watt hours or 4.18674 joules. The Btu was defined at the 

 same time as 251.996 international calories. Thus, until such a time as these 

 differences are taken care of, there will be some confusion. 



Celsius temperature scale. — The present-day designation of the scale 

 formerly known as the Centigrade scale. 



Centigrade temperature scale. — The temperature scale that divides the 

 interval between the ice point, taken as 0°C, and the boiling point of water 

 with 100°. 



Coefficient of thermal expansion. — Ratio of the change of length per 

 unit length (linear), or change of volume per unit volume (voluminal), to the 

 change of temperature. 



Color temperature 6 (T$). — The color temperature of a non-blackbody is 

 the temperature at which it is necessary to operate the blackbody so that the 

 color of its emitted light will match that of the source studied. 



Emissivity. — Ratio of the energy radiated at any temperature by a non- 

 blackbody to that radiated by a blackbody at the same temperature. The 

 spectral emissivity is for a definite wavelength, and the total emissivity is 

 for all wavelengths. 



Enthalpy. — Total energy that a system possesses by virtue of its tempera- 

 ture. Thus, where U is the internal energy, then the enthalpy = U + PV where 

 PV represents the external work. 



Entropy. — A measure of the extent to which the energy of the system is 

 unavailable. 



Fahrenheit temperature scale. — A scale based on the freezing point of 

 water taken as 32° and the boiling point of water taken as 212°. 



Graybody. — A body that has a constant emissivity for all wavelengths. 



Heat. — Energy transferred by a thermal process. Heat can be measured 

 in terms of the dynamical units of energy, as the erg, joule, etc., or in terms of 

 the amount of energy required to produce a definite thermal change in some 

 substance, as for example the energy required per degree to raise the tempera- 

 ture of a unit mass of water at some temperature. The mechanical unit of 

 heat has the dimensional formula of energy (ML 2 T~ 2 ). The thermal unit 

 (H), as used in many of these tables, is (M8) where 6 denotes a temperature 

 interval. 



Joule's equivalent (/) or the mechanical equivalent of heat. — Con- 

 version factor for changing an expression of mechanical energy into an ex- 

 pression of thermal energy or vice versa (4.1855 7/cal). 



6 Gen. Electr. Rev., vol. 47, p. 26, 1944. 



SMITHSONIAN PHYSICAL TABLES 



