132 LAWRENCE R. PROUTY AND JAMES D. HARDY 



Temperature is an indication of the thermal energy level of a 

 body. Thermal energy will flow of its own accord from a warm ob- 

 ject to a cooler one. Therefore, temperature is a property that deter- 

 mines the rate and direction of this heat flow. A large number of 

 thermal states or temperatures can be differentiated and are char- 

 acterized by definite phenomena. Any function of matter that 

 varies continuously with temperature may be used to indicate tem- 

 perature. The most useful of these include the physical phenomena 

 of expansion and change of state, and properties such as the change 

 of electrical resistance of a conductor. The Seebeck effect at the 

 junction of two dissimilar metals is of great practical importance 

 in the measurement of temperature. 



A. TEMPERATURE SCALES 



1. Arbitrary Scales 



^'arious temperature scales have been devised to characterize the 

 infinite number of thermal states between fixed points. The fixed 

 temperatures most widely used as reference points are the melting 

 and boiling points of pure substances at a constant known pressure. 

 In the scales discussed below the steam and ice points of water were 

 used as reference points. 



Celsius Scale. The first thermometric scale was devised by 

 Celsius. He assigned the value of 100 to the ice point and zero to 

 the steam point and divided the interval into 100 degrees. "De- 

 grees Celsius" is a term used in continental Europe at the present 

 time and has come to denote what English-speaking countries 

 refer to as Centigrade temperatures. 



Centigrade Scale. The Centigrade scale divides the range be- 

 tween the freezing point of water, arbitrarily called zero, and the 

 boiling point, arbitrarily called 100, at standard atmospheric pres- 

 sure, into 100 equal divisions each of which is one Centigrade degree. 

 This scale can be employed with the decimal system with facility. 

 The Centigrade scale with several fixed and reproducible equilib- 

 rium temperatures is the basis of the International Temperature 

 Scale adopted in 1927 by the General Conference of Weights and 

 Measures. The Centigrade scale is used almost exclusively in scien- 

 tific laboratories in this country at the present time, and the Fahren- 

 heit scale largely in industry. 



