42 



0. T. Sherman — A Study of Thermometers. 



Art. VI. — A Study of Thermometers ■intended to measure 

 Temperatures from lb0°-300 o C. ; by O. T. Sherman. 



It is well known that when a thermometer is heated above 

 a certain point, the mercury column is permanently displaced 

 with regard to the scale.* The position of the point depends 

 upon the constitution of the glass forming the bulb and upon 

 the previous use of the thermometer. For certain glasses des- 

 ignated by the maker as German or American soda and Cor- 

 nish the elevation upon a new thermometer begins at 111 . 

 For a flint or crystal tube the point is nearer 200°. Mills 

 records 256° as his highest observed limit, 48° as his lowest. 

 Our experience presents nothing lower than 110°, nor higher 

 than 255°. The latter point is obtained with English flint or 

 French crystal. 



By much use or long heating the displacement frequently 

 amounts to ten degrees Centigrade, and may amount to 26°. f 

 To assign corrections to points so easily displaced is evidently 

 nugatory. The Observatory has therefore hitherto confined 

 its corrections to points below that at which the ascent began. 



If now the thermometer be exposed to a high temperature 

 for some hours, the successive positions of the ice-point will 

 be found to arrange themselves in a curve similar to that in 

 the adjoining figure. Thus, for the first eighteen hours dur- 



24 36 48 60 72 



Hours of heating at 300°. 



ing which the thermometer Y. O. 80 was held at 300° Centi- 

 grade, the zero point was elevated 3°; for the second eighteen 

 hours the elevation was 2 0, 2 ; for the succeeding periods 1°*7, 

 1 0, 1, 0°'8, 0°'3 respectively.^: The elevation evidently becomes 

 less and less, and the curve becomes more nearly parallel to the 

 axis of abscissas. This same thermometer placed in a bath at 

 200° immediately after the last observation rose two-tenths of 



* Mills, Transactions Royal Society of Edinburgh, vol. xxix, part II. 



f Crafts, Comptes Rendus, 1881 and 1882. 



% "Weber, Metronomische Beitrag, No. Ill, pp. 126, 127. 



