134 LAWRENCE R. PROUTY AND JAMES D. HARDY 



extending over a wider range than liquid-in-glass thermometers, suf- 

 fers from the same limitations, i.e., the change in state of the gas and 

 the softening point of the gas container. 



All measurements should ultimately be made in fundamental 

 units such as mass, length or time, or in derived units such as energy 

 levels. The development of the absolute temperature scale makes 

 possible the measurement of temperature in terms of fundamental 

 units that are independent of the thermometric substance. This 

 scale is useful throughout the entire temperature range. It was 

 formerly believed that heat was a material substance that, when added 

 to a body, made it warmer and, when subtracted, made it cooler. 

 Experimental studies of Rumford (1798), Joule (1842), and Rowland 

 (1873) helped establish the concept that heat is a form of energy and 

 that equivalent relationships may be found between units of heat and 

 units of mechanical energy. Temperature is the property that de- 

 termines the direction of heat flow. 



From thermodynamic considerations, Lord Kelvin proposed a 

 scale of temperature that was independent of the expanding or 

 "working" substance. This scale has more meaning from a physical 

 standpoint since the zero point of the scale corresponds to conditions 

 of zero energy of the working substance and represents the lowest 

 theoretical temperature obtainable. Furthermore, it is a scale to 

 which measurements of radiant energy as well as mechanical energy 

 can be referred conveniently. The absolute scale is a purely theoret- 

 ical one but, since it corresponds closely with the gas thermometer 

 scale, at temperatures usually encountered, the absolute scale has 

 wide practical application. The size of the degree on the absolute 

 scale was chosen equivalent to the degree Centigrade. Thus: 



°K = °A = °C -f 273 



The absolute scale is particularly important in those investigations 

 in biophysics involving thermodynamics and heat transfer by radia- 

 tion. 



B. LIQUID THERMOMETERS 



1. Miscellaneous Modifications 



Alcohol Thermometers. Thermal expansion of alcohol is 

 about ten times as great as that of mercury under similar condi- 



