Thermometers are instruments for measuring temperature by the contraction or expansion of fluids in enclosed tubes. The tubes, which are of glass, have spherical, cylindrical, or spiral bulbs blown on to one end; they have also an exceedingly fine bore, and when mercury or spirit is enclosed in them these fluids, in contracting and expanding with variations of temperature, indicate degrees of heat in relation to two fixed points—viz., the freezing and boiling points of water. Care is taken to exclude all air before sealing, so that the upper portion of the tube inside shall be a perfect vacuum, and thus offer no resistance to the free expansion of the mercury. In graduating, or dividing the scales, the points at which the mercury remains stationary in melting ice and boiling water are first marked on the stem, and the intervening space divided into as many equal parts as are necessary to constitute the scales of Fahrenheit, Réaumur, or Celsius, the last being known as the Centigrade (_hundred steps_) scale, from the circumstance of the space between the freezing and boiling points of water being divided into one hundred equal parts (Fig. 16).
[Illustration: 16. Comparison of Thermometer Scales. Scale about 1/5.]
[Illustration: 17. “Legible” Scale Thermometer. Scale about 1/5.]
GRADUATION OF THERMOMETERS.—When the fluid (either mercury or spirit) has been enclosed in the hermetically sealed tube, it becomes necessary, in order that its indications may be comparable with those of other instruments, that a scale having at least two fixed points should be attached to it. As it has been found that the temperature of melting ice or freezing water is always constant, the height at which the fluid _rests_ in a mixture of ice and water has been chosen as one point from which to graduate the scale. It has been also found that with the barometer at 29·905 the boiling-point of water is also constant, and when a thermometer is immersed in pure distilled water heated to ebullition, the point at which the mercury remains immovable is, like the freezing-point, carefully marked, the tube is then calibrated and divided as shown in Fig. 16.
The zero of the scales of Réaumur and Centigrade is the freezing-point of water, marked, in each case, 0°, while the intervening space, up to the boiling-point of water, is divided, in the former case, into 80 parts, and in the latter to 100°.