Galileo's Water Thermometer. --Probably the first instrument for measuring temperature changes was constructed by Galileo at Padua, Italy, near the end of the sixteenth century. The principle employed by him was the same as that employed in the construction of modern thermometers --namely, the unequal expansion of gases, liquids, and metals with rise in temperature. Thus, Jena glass, mercury, and air expand 0.00002533, 0.0001818, and 0.00367 times their volumes at 0° C., respectively, for a rise in temperature of 1° C., or the cubical expansion of mercury is over 7 times, and the cubical expansion of air 145 times, that of glass. Galileo's thermometer was probably in the form of a large glass bulb with a long stem attached. The bulb was uppermost and the open end of the tube was immersed in a cup of water, with water also partly filling the stem. If then the temperature of the bulb was raised, the increased volume of the air that it contained depressed the column of water in the tube. Conversely, a decrease in the temperature of the bulb diminished the volume of air and allowed the water column to rise.
Mercurial Thermometer. --Similarly, when a glass bulb with a slender tube attached is filled with mercury, a rise in the temperature causes the mercury to overflow from the bulb into the tube, and the amount of this overflow is a measure of the rise in temperature. This is the form of Galileo's thermometer ordinarily used in measuring air temperature.
In order that the thermometer may respond quickly to changes in temperature it is necessary that the bulb be made as small as possible, and that it present as much surface as possible to the air. In the better grades of thermometers the bulb is therefore made cylindrical in form (Fig. 10)____________________