138 LAWRENCE R. PROUTY AND JAMES D. HARDY 



Maximum-Minimum Thermometers. IMaximum and minimum 

 thermometers indicate the highest and lowest temperatures at- 

 tained during an experimental period. One type of minimum ther- 

 mometer consists of a small glass rod immersed in alcohol. Due to 

 surface tension the glass rod is drawn along the tube as the alcohol 

 contracts with falHng temperature. On expansion of the alcohol 

 with rising temperature, the glass rod remains stationary indicating 

 the lowest temperature reached. 



A maximum thermometer is a mercurj^-in-glass thermometer 

 that may operate on the stem constriction principle (Negretti and 

 Zambra's version) as in clinical thermometers, or, alternatively, em- 

 ploys an index that remains at the highest temperature achieved. 

 This index may be a small piece of iron (Rutherford's version) or a 

 thread of mercury separated from the main column by an air bubble 

 (Phillips' version). 



2. Calibration of Mercury-in-GIass Thermometers 



The calibration of a mercury-in-glass thermometer is ordinarily 

 accomplished in the laboratory by comparison with a standard ther- 

 mometer calibrated and certified by the National Bureau of Stand- 

 ards. Thermometers with structural defects that may influence the 

 reproducibility of their readings will be calibrated upon request by 

 the thermometry section of the Bureau of Standards. These ther- 

 mometers will not, however, be certified. A certificate is supplied 

 with each thermometer calibrated and certified by the Bureau, indi- 

 cating the correction factors for each stem marking under the condi- 

 tions of calibration. The certificate also contains information for 

 calculating emergent stem corrections in case it is desired to use a 

 standard thermometer under conditions different from those of the 

 original calibration. Waidner and Mueller (i) studied the relative 

 merits of total and partial stem immersion and concluded that for 

 ordinary laboratory work more accurate measurements can be se- 

 cured by partial immersion thermometers. Thermometers gradu- 

 ated in divisions smaller than 0.5°C. should usually be standardized 

 as total immersion thermometers. Although the conditions for inter- 

 comparison of laboratory and standard thermometers may vary, 

 certain precautions should be taken to insure reproducibility in com- 

 parison measurements. Among the most important of these precau- 

 tions are: 



