Fire and Heat 



85 



38. Measurement of temperature. It is necessary in very 

 many processes of manufacturing and in the household to secure 

 and maintain a definite temperature. Since feeling is too 

 variable to be taken as a guide in such matters, an instrument 

 is necessary which will determine reliably and automatically 

 temperature and its changes. Such an instrument is the 

 thermometer. With its aid we can find exactly the 

 temperature of any substance, such as the air, 

 water, fire, earth, or any living organism. 



Construction and action of the thermometer. 

 Examine a thermometer (Fig. 23). The usual 

 household type is simple in construction, made of 

 a glass tube with a very small but uniform bore 

 and with a small bulb at one end. The bulb is 

 filled with mercury which usually reaches some dis- 

 tance into the tube. The tube is either graduated 

 or fastened to a graduated scale. Measure the 

 spaces. Are they equal? The graduated distances 

 are determined by means of what are known as 

 the fixed points, which are the freezing and boiling 

 points of water. When the fixed points are deter- 

 mined for any thermometer, the distance between 

 them may be easily divided into equal parts. How 

 many spaces are there between the fixed points 

 of the thermometer under examination? 



You will observe that when the mercury in the bulb is 

 warmed, as with the hand, the height of the column in the tube 

 changes. It rises to a higher level, which may be read on the 

 scale as degrees. When the bulb is cooled, the mercury column 

 falls. The behavior of mercury under the influence of heat 

 and cold is a fact of great importance in understanding how 

 temperature is measured. Air was found to behave in similar 

 fashion, page 64 ; it expanded with warmth and contracted 

 with cold. It is known that all gases behave in the same way. 



Water is not used in thermometers because it does not expand 



FIG. 23. 

 A household 



