THE MEASUREMENT OF HIGH TEMPERATURES 231
One of the latest forms of the special mercury thermometer for reading temperatures up to 540° is the Hohmann and Maurer instrument as made by the Cambridge Scientific Instrument Company. The author has found this type of thermometer particularly suitable for recording the temperatures of benzol stills and similar apparatiis, and it is accurate to about 1 per cent. The principle of its operation will be readily understood by reference to Fig. 130. A is a steel bulb, connected by means of a steel capillary tube B, to a special form of steel Bourdon spiral C. The whole system is filled with mercury, and changes of temperature of the bulb give rise to corresponding changes of pressure in the system which are magnified and
recorded or indicated by means of a simple pen or pointer mechanism in connection with the Bourdon spiral.
OPTICAL PYROMETERS
Witkin recent years a number of optical pyrometers have been developed which. compare different colours with that of the heated obj eet. These, however, would appear to depend largely upon the individual in whose hånds they are placed, for it is seldom that two operators obtain exaetly similar results. Another form of instrument is that in which a glass prism graduated in depth of colour is employed, differing depths of colour being required to eclipse the light emitted by bodies in differing degrees of temperature. For retort-house and similar operations the author has found this type of instrument particularly suitable in spite of the faet that it is, to some extent, dependent upon tlie human element. When extreme accuracy