Every thermometer, especially if intended for use above the boiling point, should be suitably annealed before it is used. If this is not done, the true melting point and also the "fundamental interval", that is, the interval between the melting and the boiling points, may change considerably. After continued use at the higher temperatures also, the melting point will change, so that the thermometer must be calibrated occasionally to insure accurate readings. TABLE 3 COMPARISON OF THERMOMETER SCALES +---------------+----------+----------+----------+ | |Fahrenheit|Centigrade| Réaumur | +---------------+----------+----------+----------+ |Absolute Zero | -459.64 | -273.13 | -218.50 | | | 0 | -17.78 | -14.22 | | | 10 | -12.22 | -9.78 | | | 20 | -6.67 | -5.33 | | | 30 | -1.11 | -0.89 | |Freezing Point | 32 | 0 | 0 | |Maximum Density| | | | | of Water | 39.1 | 3.94 | 3.15 | | | 50 | 10 | 8 | | | 75 | 23.89 | 19.11 | | | 100 | 37.78 | 30.22 | | | 200 | 93.33 | 74.67 | |Boiling Point | 212 | 100 | 80 | | | 250 | 121.11 | 96.89 | | | 300 | 148.89 | 119.11 | | | 350 | 176.67 | 141.33 | +---------------+----------+----------+----------+ F = 9/5C+32° = 9/4R+32° C = 5/9(F-32°) = 5/4R R = 4/9(F-32°) = 4/5C As a general rule thermometers are graduated to read correctly for total immersion, that is, with bulb and stem of the thermometer at the same temperature, and they should be used in this way when compared with a standard thermometer. If the stem emerges into space either hotter or colder than that in which the bulb is placed, a "stem correction" must be applied to the observed temperature in addition to any correction that may be found in the comparison with the standard. For instance, for a particular thermometer, comparison with the standard with both fully immersed made necessary the following corrections: _Temperature_ _Correction_ 40°F 0.0 100 0.0 200 0.0 300 +2.5 400 -0.5 500 -2.5 When the sign of the correction is positive (+) it must be added to the observed reading, and when the sign is a negative (-) the correction must be subtracted. The formula for the stem correction is as follows: Stem correction = 0.000085 × n (T-t) in which T is the observed temperature, t is the mean temperature of the emergent column, n is the number of degrees of mercury column emergent, and 0.000085 is the difference between the coefficient of expansion of the mercury and that in the glass in the stem.