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19_16 - EMERGENT STEM CORRECTION FOR LIQUID-IN-GLASS...

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Unformatted text preview: EMERGENT STEM CORRECTION FOR LIQUID-IN-GLASS THERMOMETERS Accurate thermometers are calibrated with the entire stem immersed in the bath which determines the temperature of the thermometer bulb. However, for reasons of convenience it is common practice when using a thermometer to permit its stem to extend out of the apparatus. Under these conditions both the stern and the mercury in the exposed stern are at a temperature different from that of the bulb. This introduces an error into the observed temperature. Since the coefficient of thermal expansion of glass is less than that of mercury. the observed temperature will be less than the true temperature if the bulb is hotter than the stem and greater than the true temperature, providing the thermal gradient is reversed. For exact work the magnitude of this error can only be determined by experiment. However, for most purposes it is sufficiently accurate to apply the following equation which takes into accoant the difference of the thermal expansion-of glass and mercury: T. = '1'a + FthTo—T...) T = corrected temperature T. = observed temperature T... = mean temperature of exposed stem. The mean temperature of the exposed stem may be determined by fastening the bulb of a second thermometer against the midpoint of the exposed liquid column. L = the length of the exposed column in degrees above the surface of the substance whose temperature is being determined. F = correction factctr. Fer approximate work and when the tiquid in the thermometer is mercury a value for F of 0.00016 is generally used. For more accurate work with mercury filled thermometers values as given in the following table are used. For thermometers filled with organic liquids it is customary to use 0.001 for the value of F. Values of F for various glasses Corning Corning Corning J can Jena Tm‘C. m4} 881:!) 8810 16 III 59 ill 50 0.000157 0.000166 0.000l56 0.000153 0.000164 150 0.000159 0.00016? 0.000157 0.000158 0.000165 250 0.000163 0.000168 0.000161 0.00016l 0.000170 3.50 0.000168 0.000173 0.000166 — 0.000177 ...
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