THE THEKMOMETER 



761 



A 



latitudes, or as can be produced artificially, it freezes, 



and is consequently no longer available for indicating 



changes of tempe.rature. For such ^ 



low temperatures a thermometer filled 



with alcohol must be used, for alcohol 



does not freeze at any temperature 



hitherto observed. 



A thermometer consists of a stem 

 formed of a capillary glass tube with 

 thick sides, at the end of which a cy- 

 lindrical or spherical bulb, with very 

 thin sides, is blown. The bulb and n 

 part of the stem are filled with mer- 

 cury, the remaining space within the 

 stem being a vacuum. The expansion 

 is measured on a scale, graduated 

 either on the stem itself or on a frame 

 of wood, glass, or metal, to which the B 

 stem is attached. In thermometers 

 which are used for liquids the stem 

 and scale are generally either sur- 

 rounded by an outer tube, which is 

 fused below to the bulb, as A and B 

 in fig. 381, the scale being graduated 

 on a strip of paper or on white glass, 

 or the stem is made thicker than the 

 diameter of the bulb, and the scale is 

 engraved on the stem itself, as in C 

 and J) of fig. 381. Thermometers of the latter 

 form are frequently used in physical experiments, 

 because they can be easily introduced into narrow 



FIG. 381 



(A, C, 3 real size ; 

 B, D, real size). 



