HEAT. 



A thermometer should be used when possible in the position in which 

 its fixed points were marked. If, for instance, they were marked with 

 the stem vertical, then in the horizontal position the internal pressure 

 on the bulb due to the column of mercury in the stem is removed and 

 the bulb contracts slightly, indicating too high a temperature. 



Corrections can, however, be determined by direct experiment and 

 can be applied to the observed reading. With the most sensitive instru- 

 ments it is also necessary to take into account the varying pressure on 

 the outside of the bulb due to change of atmospheric pressure or depth 

 of immersion in a liquid. 



Limits of Accuracy. With different thermometers made of the same 

 kind of glass and carefully graduated, the indications of a given tempera- 

 ture should agree to within about 0'01 C. Different kinds of glass have 



1334 

 13 



12- 

 II- 



n- 



9- 



tf- 



7- 

 6- 



5- 

 4~ 

 3- 

 i- 

 I- 

 O'F.P 



Asymptote 13 54 



April I8W 



1844 47 SO S3 56 39 62 <35 P8 71 74 77 80 83 86 



FlG. 5. Joule's Observations on the Alteration of the Freezing Point in Thermo- 



. i 



meters, and the Comparison with the Curve /=13'94:-9 - 5lV^ 3 



different expansions and, though their indications at and 100 will 

 agree, intermediate indications may differ by, perhaps, more than 0'1C. 

 Above 100 their indications may differ much more widely than this. In 

 recent years very careful attention has been paid to the qualities of 

 different glasses for thermometer purposes and to the methods of 

 correction to be employed to make the readings of different thermo- 

 meters give the same value of the same temperature.* 



It is to be hoped that, as a result of these investigations, the quality 

 of glass used will be improved, and that scientific thermometer makers 

 will everywhere use the same glass, the same mode of marking the fixed 

 points, and the same mode of graduating, so that the indications of 

 different instruments may be immediately compared. 



Range of the Mercury-Glass Thermometer. There are limits 



* In Dr. Chree's " Notes on Thermometers," in the Philosophic"/ .!/</</" -.me, xlv. 1898 

 p. 205, will be found a description and full discussion of the various corrections 



