nt is marked zero, and the boiling point is
marked 100 in the centigrade and 80 in the Reaumur. Each of the 180, 100
or 80 divisions in the respective thermometers is called a degree.

Table 3 and appended formulae are useful for converting from one scale
to another.

In the United States the bulbs of high-grade thermometers are usually
made of either Jena 58^{III} borosilicate thermometer glass or Jena
16^{III} glass, the stems being made of ordinary glass. The Jena
16^{III} glass is not suitable for use at temperatures much above 850
degrees Fahrenheit and the harder Jena 59^{III} should be used in
thermometers for temperatures higher than this.

Below the boiling point, the hydrogen-gas thermometer is the almost
universal standard with which mercurial thermometers may be compared,
while above this point the nitrogen-gas thermometer is used. In both of
these standards the change in temperature is measured by the change in
pressure of a constant volume of the gas.

In graduating a mercurial thermometer for the Fahrenheit scale,
ordinarily a degree is represented as 1/180 part of the volume of the
stem between the readings at the melting point of ice and the boiling
point of water. For temperatures above the latter, the scale is extended
in degrees of the same volume. For very accurate work, however, the
thermometer may be graduated to read true-gas-scale temperatures by
comparing it with the gas thermometer and marking the temperatures at 25
or 50 degree intervals. Each degree is then 1/25 or 1/50 of the volume
of the stem in each interval.

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| Reaumur |
+---------------+----------+----------+----------+
|Absolute Zero | -459.64 | -273.13 | -218.50 |
| | 0 | -17.78 | -14.22 |
| | 10 | -12.22 | -9.78 |
| | 20 | -6.67 |