Page:The American Cyclopædia (1879) Volume XV.djvu/729

 THERMOMETER 699 the mercury. The instrument which shows this difference in expansions is known as the mercurial thermometer. It consists of a tube of very small interior diameter, terminating in a bulb or reservoir. The bulb and a portion of the tube are filled with mercury, and with an increase or a diminution of temperature the mercury will rise or fall in the tube ; and the position of the mercury in the tube can be noted on a scale of equal parts either etched on the tube or marked on the surface or a plate to which the tube is attached. Mercury has several advantages as a thermometric sub- stance. The successive increases in its volume for equal and successive additions of tempera- ture, indicated by the air thermometer (see PYROMETER, vol. xiv., p. Ill), are quite uni- form ; especially is this the case when we use the differential expansion of mercury and ordinary glass. The ordinary thermometer when constructed with care is trustworthy in the measure of temperatures up to 300 0. Up to 100 0. mercurial thermometers made of any kind of glass indicate almost exactly the same temperatures as those given by the air thermometer. Another advantage of mer- cury is that it does not freeze above the low temperature of 40 0., and does not boil below 360 C. But the mercury thermometer only gives accurate indications between 35 and +300 C. For temperatures above 300 0. some form of pyrometer must be used. Mercury has a low specific heat, and this prop- erty combined with its high conductivity causes it rapidly to indicate the changes in the tem- perature of surrounding bodies or of the medi- um in which it is immersed. Construction of the Mercurial Thermometer. The tube of the thermometer should be of uniform calibre throughout its whole interior. To ascertain whether this is the case, a short column of mercury is introduced into the tube ; and if its length remains the same when it is moved throughout the length of the tube, we may be sure that the tube has a uniform bore, and hence that equal amounts of expansion of the mercury will cause equal additions in the length of the mercurial column in the tube. Since tubes of uniform bore are very rare, it is gen- erally necessary to calibrate the tube before its graduation. This is done by etching on the tube a scale of equal parts, and then, from observations on the different lengths occupied by a column of mercury which is made to pass through the tube, forming a table which gives the temperatures corresponding to the arbitra- ry divisions on the tube. A bulb is now blown on the tube, and this bulb and a portion of the tube are filled with mercury as follows : The air in the bulb is heated while the open end of the tube dips into mercury. The heat having been withdrawn, the air in the bulb contracts and the mercury rises in the tube and partly fills the bulb. To the open end of the tube a funnel containing mercury is adapted, and the mercury in 'the bulb is boiled and thus expels all air and moisture from the instru- ment, which on cooling necessarily fills com- 3letely with mercury. The bulb is now placed n some fluid heated to a few degrees above the highest temperature which the thermom- eter is intended to measure, and when the mercury ceases to overflow the open end of the tube is sealed with a blowpipe flame. In order to graduate the instrument, the bulb and part of the tube are surrounded with melting ice, and when the top of the mercury column has remained some time stationary, its posi- tion is marked by means of a line, or a note is made of this position, referred to the arbitrary scale etched on the tube. The point on the thermometer determined as above is designa- ted as 0, or zero degree, on the thermometers known as centigrade (Celsius) and Reaumur, and as 32 on the Fahrenheit system of grad- uation. To determine a higher point on the thermometer, the instrument is placed in the interior of a metallic vessel with double walls, between which circulates the steam from wa- ter boiling in the bottom of the vessel. When the top of the mercury column in the ther- mometer has become stationary its position is marked on the tube. The boiling point of water is constant at the same atmospheric pressure, and when the barometric column has a height of 29 -922 inches or 760 milli- metres, the boiling point of water is desig- nated as 100 on the centigrade thermome- ter, 212 on the Fahrenheit, and 80 on the Reaumur. Hence, between the melting point of ice and the boiling point of water there are 100 equal degrees in the centi- grade graduation, 180 in the Fahrenheit, and 80 in the Reaumur. To convert the indications of one of these thermometers into those of the other two, we have the following formula, in which F, C, and R denote equivalent temperatures expressed in de- grees of Fahrenheit, centi- grade, and Reaumur, respec- tively : C=J-R=f(F-32) R=|C=|(F-32) Fig. 1 shows a thermometer graduated according to the three systems. A few weeks after a thermometer has been made and graduated it may be observed that the mercury will not quite descend to the melting point of ice when the instrument is immersed in pounded ice. It has been found that this " elevation of the zero point, as it is called, goes on gradually for about two years after the thermometer has been con- FIG. 1. Thermom- eter with Fahren- heit, Reaumur, and Centigrade Scales.