136 LAWRENCE R. PROUTY AND JAMES D. HARDY 



mercury column must he made to travel over a greater distance along 

 a stem of small bore from a comparatively large mercury bulb. This 

 requires either a very long stem or an enlargement in the upper end of 

 the stem into which the mercury can be driven by heat. Accuracy, 

 the extent to which the thermometer indicates true temperature 

 values, can be estimated by the correlation between the temperatures 

 measured by that thermometer and a precise thermometer in abso- 

 lute units. It is affected by thermal expansion 

 coefficients of the glass, sensitivity, uniformity of 

 bore, ease and skill with which the scale can be 

 read, precisepess of calibration, and many other 

 factors. 



Any thermometer indicates primarily its own 

 temperature as affected by the environment. A 

 thermometer that is sensitive to small temperature 

 - -^"scafe *^ increments will, because of the comparatively large 

 mass of the bulb, have a greater tendency to modify 

 the temperature of its environment. Hence, in this 

 regard, a sensitive thermometer may be less ac- 

 curate than one of smaller mass and it may be 

 slower to respond to thermal changes. 



Beckmann Differential Thermometers. T(; 

 obtain a thermometer that is both sensitive and 

 accurate, Beckmann devised one (Fig. 1) which, 

 although it does not permit absolute tempera- 

 tures to be read, does give differences in tempera- 

 Bulb ^^^^ ^'^^^^ ^^^ error not exceeding 0.001 to 0.002°C. 

 If this type of thermometer were made in the 



T^. 1 Tj 1 ordinary manner, the stem would have to be in- 



l^ig. 1. Beckmann .- ' 



thermometer. conveniently long or a number of thermometers 

 for different temperatiu'e ranges would be required. 

 In the Beckmann thermometer the amount of mercury in the bulb, 

 and consequently the temperature range of the thermometer, can 

 be altered by driving some of the mercury into a reservoir connected 

 to the upper end of the stem. It is necessary to "set" the ther- 

 mometer before using so that the end of the mercury thread is on 

 the scale in the desired temperature range. This is done by sus- 

 pending the thermometer in a beaker of water whose temperature 

 is regulated with the aid of an ordinary thermometer at a level 

 two or three degrees higher than the highest temperature to be met 



