Chapter 7-PRINCIPLES OF MEASUREMENT 



Where special requirements exist, special 

 types of liquid-in-glass thermometers are used. 

 For example, maximum and minimum indica- 

 ting thermometers are used in magazines aboard 

 ship, for weather observations, and for various 

 other applications where it is necessary to 

 know the highest and the lowest temperatures 

 that have occurred during a certain interval 

 of time. One type of maximum indicating ther- 

 mometer is shown in figure 7-4, and one type 

 of minimum indicating thermometer is shown 

 in figure 7-5. 



The maximum indicatingthermometer shown 

 in figure 7-4 is a mercury thermometer with 

 a special constriction in the bore. When the 

 temperature rises, the pressure of the expand- 



ing mercury in the bulb forces mercury past 

 the constriction in the bore. When the tempera- 

 ture falls, the mercury does not return to the 

 bulb. Why? Even if the thermometer were in an 

 upright position, the constriction in the bore 

 would prevent the normal return flow of mercury 

 by gravity in addition, the maximum indicating 

 thermometer is mounted with the bulb a few 

 degrees above the horizontal position, so that the 

 mercury column slopes downward from the con- 

 striction. Thus the thermometer always indi- 

 cates the highest temperature that has been 

 reached since the instrument was last set. Ex- 

 pansion and contraction of the mercury in the 

 bore above the constriction does occur with 

 temperature changes, but it is so slight as to 



SUPPORT 



CONSTRICTION 



INVERT FOR 

 SETTING 



LOWER CAREFULLY 



TO VERTICAL 

 POSITION TO READ 



Figure 7-4.— Maximum indicating thermometer. 



CURRENT CURRENT & MIN. 



TEMPERATURE 



5.65A 



NDEX 



Figure 7-5.— Minimum indicating thermometer. 



131 



5.65B 



