Page:Handbook of Meteorology.djvu/191

 The Thermometer.—The ordinary thermometer consists of a bulb or reservoir fused to the end of a glass tube or stem. The tubing from which the stem is drawn is a wedge-shaped prism with a strip of white enamel fused to the convex surface. When the prismatic tube is drawn out into thermometer stems the wedge angle becomes a lens which magnifies the fine thread of mercury; the enamel becomes an opaque, white background against which the thread of mercury is plainly visible. The bulbs of ordinary thermometers are commonly blown at the end of the drawn tube. Those of the best thermometers are made of a specially constructed glass and are fused to the end of the drawn tube.

Most solids, in cooling from fusion or from intense heating, suffer what is known as “hysteresis”—that is, molecular changes continue for a considerable time. These changes alter the size of the bore of the tube. In order to overcome them, the tubes of thermometers of the highest grade are laid away to “season” or “temper” for a period of two years. The shrinkage of an unseasoned tube is likely to cause the readings to register as much as 6 degrees too high.

The bore of the thermometer is microscopic in diameter; in thermometers graduated to fractions of a degree, it may be less than o.ooi inch; ordinarily it is from 0.002 inch to 0.005 inch. In the construction of precision thermometers the bore is measured under the microscope, and a bulb of hard glass of the required size is fused to the end of the tube. Cylindrical bulbs are preferable to spherical bulbs; they present a greater surface and therefore are more sensitive. The expansion and contraction of the glass with changes of temperature is somewhat greater than with spherical bulbs, but thermometer scales are compensated for this correction.

The bulb of the thermometer is usually filled with mercury at the time it is fused to the stem. While hot, the open end of the stem is inserted in a vessel containing pure mercury. As the air in the bulb cools, its contraction causes a small quantity of mercury to be forced through the bore of the stem into the bulb. The mercury in the bulb is then heated to its boiling point and the open end of the stem again dipped into the mercury. This process is repeated until both bulb and steam are completely filled.