230
MODERN GASWORKS PRACTICE
Fig. 129.—The Steinle Thermometer.
Fig. 128.—
Spiral of
Steinle Thermometer,
rolled into nine coils.
consist of fusible mixtures—generally clay, sand, and special fluxes—of known melting-points. The fluxes used mainly consist of soda or lead oxide.
As regards special mercury thermometers, it is well known, of course, that mercury under atmospheric pressure boils at 360°,1 and above about 260°’ its reliability is uncertain. If, however, the mercury is main-tained under pressure its boil-ing-point is raised, so that thermometers on this principle have b e en constructed to measure temperatures up to 540° (1,000° F.). One such instrument is the Steinle,. shown in Fig. 128. It consists of a strong, mild steel cylinder A, which is closed at one end and tapers off at the other end to a very fine bore which con-nects at B with a steel spiral spring C. The spiral is made from a solid drawn steel tube with thin walls rolled flat until the sides meet within T^-T inch. The container and spiral spring are entirely filled with. mercury» after which the upper end of the spring is hermetically sealed. When, owing to a rise in temperature, the mercury expands, the pressure causes the spiral partlyto uncoil, and conversely. The rotary motion of the spring may, therefore, be transferred by means of suitable gearing to a dial hand working over a specially calibrated scale. The spiral of average size is about 21-inch long before coiling, and is.
One form of the complete instrument is shown in Fig. 129-All temperatures are in degrees Centigrade unless otherwise stated.