Page:Handbook of Meteorology.djvu/192

 The mercury and the tube are apt to contain a minute quantity of moisture. If this remains it is pretty certain to cause a broken column of mercury in the tube, thereby rendering the thermometer imperfect. To prevent this, the filled tube is kept for some hours at a temperature above the boiling point of water. The “roasting” requires care and experience.

When the roasting process is completed the bulb is again heated and as soon as the mercury is expanded to the top of the stem the latter is sealed, leaving an angle or “hook” at the top. The hook holds the tube fast to the scale.

The tube is now ready to be graduated. For this purpose it is placed successively in brine at 2°, melting ice at 32°, and water baths at 62° and 92°. The position of the top of the column is marked for each temperature and usually for each tenth degree of the scale. An engine ruling machine divides each division into 30 parts. The division marks are also scaled below 2° and above 92°. The metal scale is subdivided according to the marks on the stem.

Several manufacturers produce three grades of instruments. On those of the first class minute spots may be found at the various testing temperatures. If the marks do not appear, the thermometer is not trustworthy for precision purposes. Weather Bureau thermometers are specially tested, and a certificate showing the error for each 10 degrees accompanies each instrument. A thermometer which does not comply with these requirements is not a standard instrument.

Thermometers of the second grade are not engraved on the stem; the divisions are on the metal scale to which the tube is attached. The tubes are seasoned; but trifling inaccuracies unfit them for use where precision is required. They are sufficiently accurate for ordinary uses.

Thermometers of the third grade are “rejects” and rarely bear the maker’s name. The inaccuracy is always more than 1 degree, and it is likely to vary in different parts of the scale. In many instances these thermometers are sold to jobbers and retailers who stamp fictitious names on them. Experience and practise will usually enable one to discover the maker. The scope of thermometer scales varies greatly. Practically every industry employs temperature measurements which require specially constructed thermometers.