V. TEMPERATURE DETERMINATIONS 137 



in the experiment. If the top of Ihe mercury cohmm of the 

 Beckmann thermometer does not rise to the scale {i.e., too httle 

 mercury in the bulb and too much in the reservoir) the thermometer 

 should be placed in a second bath at a temperature sufficiently 

 high to cause the mercury to pass up and form a small drop at the 

 upper end of the capillary. The thermometer is then inverted and 

 tapped to collect the merciuy in the reservoir at the end of the 

 capillary. Next, the thermometer is returned without shaking to 

 the vertical position, and replaced in the first bath. At this time, 

 the mercury in the bulb will contract and draw in some of the mer- 

 cury from the reservoir. When the thermometer has become fully 

 adjusted to the temperature of this bath the upper end of the ther- 

 mometer is struck against the palm of the hand in order to break off 

 the excess mercury from the end of the capillary. The thermometer 

 is then tested in the temperature range of the experiment. If the 

 mercury column rises above the scale, too much mercury has been 

 delivered to the bulb; some of it must be driven into the reservoir and 

 a small amount shaken off from the end of the capillar j\ Many 

 Beckmann thermometers are calibrated so that each of these drops 

 shaken off from the end of the capillary will lower the temperature 

 range of the scale by approximately one or two degrees. These 

 drops of mercury may be left in the glass tubing without returning 

 them either to the bull) or reservoir. Temperature readings are not 

 affected by so doing. 



Clinical Thermometers. A clinical thermometer is essentially a 

 maximum-reading, mercury-in-glass thermometer. It is made self- 

 registering by means of a constriction in the bore about 1 cm. above 

 the bulb. This permits mercury to expand upward freely onto the 

 scale as the temperature rises but prevents return of the mercury 

 to the bulb except by vigorous shaking. A clinical thermometer 

 in which the constriction is too small will have the mercury expand 

 upward in a series of jerks and, if too large, will have the mercury 

 retreat into the bulb. 



Clinical thermometers usually have an etched stem, and cover a 

 range of 10°F. (96 to 1()6°F. in intervals of 0.2°) or 6°C. (35 to 41°C. 

 in 0.1° intervals). In order to minimize thermal response time the 

 bulb is made small and consetjuently the bore must be quite fine. 

 To facilitate reading a white backing is placed behind the capillary 

 (bore) and the clear glass front is molded in the form of a magnifying 

 lens to increase the apparent ^^^dth of tlie mercury column. 



