50 EXPERIMENTAL GENERAL SCIENCE 



we turn Fahrenheit degrees to Centigrade, we must subtract 32 

 degrees before multiplying by % and, in the reverse process, we 

 add 32 after we have multiplied by %. A convenient ex- 

 pression of this method is as follows: (C. X %) + 32 = F. 

 and (F. 32) X % = C. For temperatures below zero Centi- 

 grade, 32 degrees must be subtracted after the change to Fah- 

 renheit is made, but when changing in the other direction 32 

 must first be added for all temperatures below zero Fahrenheit. 

 50. Other Thermometers. Long before the temperature 

 of absolute zero is reached, alcohol and mercury become solid 

 and are therefore not adapted to measuring extremely low 



FIG. 17. Maximum and minimum thermometers. (Duff.) 



temperatures. In the other direction, there are temperatures 

 much higher than boiling water, boiling alcohol, or even boiling 

 mercury. For measuring such temperatures, other thermom- 

 eters are needed. One instrument makes use of nitrogen or 

 hydrogen gas. Since the pressure of a gas is proportional to its 

 temperature, any difference in pressure may be translated into 

 differences of temperature. The clock thermometer indicates 

 the temperature by means of a pointer moving over a dial. 

 The motion is given to the pointer by strips of two different 

 metals fastened together. These metals have different rates 

 of contracting and expanding and thus bend each other back 



