V. TEMPERATURE DETERMINATIONS 139 



(1) Thermal equilibrium between the standard thermometer and the 

 comparison thermometer is essential. This may be provided for by a well 

 stirred water bath. 



(2) Before testing, the mercury column in both thermometers should be 

 raised to the top of the stem before heating in order to insure continuity of 

 the mercury column. Gentle tapping, with heating and cooling the ther- 

 mometer bulb, will assist in joining of a separated column. 



(5) It is generally desirable to make a comparison under conditions of in- 

 tended use of the laboratory thermometer. 



(4) Calibrate slowl)^ allowing ample time for establishing thermal equilib- 

 rium at several points in the desired temperature range. Calibration from 

 low to high temperature is safer and gentle tapping of the thermometer 

 stems will prevent slight sticking of the mercury column. 



(5) Parallax errors in reading can be avoided by use of a good ther- 

 mometer reader. 



(6) Correction factors are then recorded in tabular form or graphed with 

 temperature on one axis and correction factors on the other. Few thermome- 

 ters have bores of uniform diameter throughout their length. Thus, the cor- 

 rection factor will vary at the intervals chosen for the calibration. Interpola- 

 tions are made for corrections between calibrating points. 



Beckmann thermometers, in addition to the above calibration fac- 

 tors, must be corrected for a "setting factor" if used in temperature 

 ranges above or below that for which they are calibrated. This is 

 necessary because the quantity of mercury in the bulb is more or 

 less than the amount it contained during calibration. 



Clinical thermometers may be calibrated in the same manner as 

 any other mercury-in-glass thermometer but it is usually more im- 

 portant to test the thermometer at least once a year to see if it is a 

 ''retreater." Because of an over-sized constriction in the stem, the 

 mercury column retreats into the bulb upon withdrawal of the ther- 

 mometer from the mouth, rectum, axilla, or skin. Busse (S) recom- 

 mends testing the thermometer by holding it vertically in a beaker of 

 water at a temperature of about 39.4°C. while stirring the water with 

 the bulb. The reading is noted and the thermometer removed and 

 read again while still vertical. If the second reading is more than 

 0.1 °C. lower than the initial reading on several successive tests, the 

 thermometer should be discarded as a "retreater." CUnical ther- 

 mometers showing a break in the mercury column above the con- 

 striction probably have air in the bore and should also be discarded. 



The Bureau of Standards accepts as tolerances for clinical ther- 

 mometers readings that are correct within 0.1 °C. at 38°C. and 0.2° 



