AEROGRAPHER'S MATE 3 & 2 



affording an easy and accurate measure of 

 temperature. Welded strips of two dissimilar 

 metals are called bimetallic strips. 



There are other methods, such as measuring 

 the electrical resistance of a substance 

 (resistance changes with temperature; there- 

 fore, temperature can be found by finding the 

 resistance), spectral analysis, measurements 

 of the speed of sound, and measurement of 

 electromagnetic radiation. 



Temperature measurement devices used 

 most extensively are those dealing with a 

 confined substance (alcohol or mercury), 

 differences in linear expansion (bimetallic strip), 

 and changes in electrical resistance (radiosondes, 

 AN/GMQ-14, and AN/GMQ-29). 



Absolute Scale (Kelvin) 



Another scale in wide use by scientists in 

 many fields is the absolute scale or Kelvin 

 scale. It was developed by Lord Kelvin of 

 England. On this scale the freezing point of 

 water is 273° A (or K), and the boiling point 

 of water is 373° A (or K). The absolute zero 

 value is considered to be a point at which 

 theoretically no molecular activity exists. This 

 places the absolute zero at a minus 273° on 

 the Celsius scale, since the degree divisions 

 are equal in size on both scales. The absolute 

 zero value on the Fahrenheit scale falls at 

 minus 459,6° F. 



Scale Conversions 



TEMPERATURE SCALES 



Long ago it was recognized that uniformity 

 in the measurement of temperature was essential. 

 It would be foolhardy to rely on such subjective 

 judgments of temperature as "cool," "cooler," 

 and "coolest"; therefore, arbitrary scales were 

 devised. Some of them are described in this 

 section. They are Fahrenheit, Celsius, and 

 absolute (Kelvin) scales. These are the scales 

 in use by the Naval Weather Service as well 

 as most meteorological services of all the 

 countries in the world. (See table 12-2.) 



Fahrenheit Scale 



The Fahrenheit scale was invented by 

 Gabriel Daniel Fahrenheit about 1710. He was 

 the first to use mercury in a thermometer. 

 The Fahrenheit scale has 180 divisions or 

 degrees between the freezing point (32° F) and 

 boiling point (212° F) of water. 



Celsius Scale 



The Celsius scale was devised by Anders 

 Celsius during the 18th century. This scale 

 has reference points with respect to water 

 of 0° C for freezing and 100° C for boiling. 



It should be noted that many publications 

 still refer to the "centigrade temperature 

 scale". Although different in name, it is the 

 same as the Celsius temperature scale now 

 in universal use. 



Since all three scales are used in the Naval 

 Weather Service, it is often necessary to change 

 the temperature value of one scale to that 

 of another scale. Generally a temperature 

 conversion table is used (Table 12-2) or a 

 temperature computer. If these are not available, 

 the Aerographer must then use one of the 

 following mathematical methods to convert one 

 scale to another. 



Mathematical Methods 



It is helpful to note that there are 100 

 divisions between the freezing and boiling 

 points of water on the Celsius scale, whereas 

 there are 180 divisions between the same 

 references on the Fahrenheit scale. Therefore, 

 one degree on the Celsius scale equals nine- 

 fifths degree on the Fahrenheit scale. In 

 converting Fahrenheit values to Celsius values 

 the formula is: 



C = (F - 32») x J- 



In converting Celsius values to Fahrenheit 

 values the formula is: 



-(■M 



+ 32° 



One way to remember when to use 9/5 

 and when to use 5/9 is to keep in mind that 

 the Fahrenheit scale has more divisions than 

 the Celsius scale. In going from Celsius to 

 Fahrenheit, multiply by the ratio that is larger; 



262 



