38 



MEASUREMENTS 



•220 

 100 -^ 



1e-200 



90 -$ 



80 



E=- 180 



70 

 60 

 50 

 40 

 30 -^ 



20 -A\ 



10 ^r 



160 



i-— 140 



The amount of heat depends upon the mass of the body, and under a 

 given set of conditions a given body of a certain material must contain 

 the same amount of heat. Heat is rarely measured directly, but instead 

 the amount of heat in a body is calculated from other measurements. 



Even the calorimeter, which comes close to measuring 

 heat, actually measures changes in temperature of a 

 known amount of water or other material. Heat meas- 

 ured in this way is expressed in calories, one calorie 

 being the heat required to raise the temperature of 

 one gram of water one Celsius degree. Calculating 

 heat in joules from electrical measurements is usually 

 more precise than calorimetry. The joule, when used 

 for heat, is the energy given off in one second when 

 a current of one ampere flows through a resistance of 

 one ohm. Thus heat can be related quite directly to 

 electrical quantities. 



Temperature is a measure of the "concentration" of 

 heat, that is, the amount of heat per unit of material. 

 The temperature of a uniform object is independent 

 of its size. The Celsius (centigrade) temperature 

 scale is used almost exclusively in biology. Degrees 

 on the Absolute or Kelvin scale are the same size as 

 the centigrade degrees, but 0° K is about —273° C. 

 If temperatures must be converted from Celsius to 

 Fahrenheit, the easy way is to carry a diagram on 

 which the scales are printed side by side, as in Fig. 

 4-2. Alternatively, remember a few reference points; 

 e.g., 20° C = 68° F, 37° C = 98.6° F, 40° C = 104° 

 F, and 5 Celsius degrees equal 9 Fahrenheit degrees. 

 As a last resort, 



-i^ 



-10 -4[ 



-20 -]l 



-30 -^ 

 -40 



120 



100 



80 



60 



40 



20 







^-20 



^^-40 



Fig. 4-2. Celsius 

 (centigrade) and 

 Fahrenheit tem- 

 perature scales. 



tF = ltc + 32, 



an{ 



tc = -^ Of 



32). 



The familiar way of measuring temperature with a 

 thermometer depends upon the thermal expansion of 

 mercury. The amount of expansion per degree is constant over a wide 

 range of temperatures. Temperatures below the range where mercury 

 can be used require some other liquid, such as an alcohol or a hydro- 

 carbon. 



