TEMPERATURE. S 



Or, if a brass rod (Fig. 2), exactly fits into a space in a brass plate 

 when cool, it can no longer be inserted into the space after it is 

 heated, if the plate remains cool. 



We might use the expansion of a brass rod to give us a scale 

 of temperature, if we had a 

 sufficiently delicate and simple 

 method of measuring its length. 



We might call the tempera- 

 ture of the brass when placed 

 in ice-cold water, 0, and de- 

 scribe its temperature as rising 

 1 for every increase in length 

 of TOOOOO but the difficulty 



of measuring such a small in- FIG. 1. Expansion of a Heated Rod. 



crease as would occur for any 



ordinary rise of temperature would render such a scale of little practical 



value. 



We therefore make use of the facts that the expansion of liquids by 



heat is usually much greater, and that of gases enormously greater, than 



that of solids. 



If a glass flask is filled with water, and then closed by a cork with a 



tube of narrow-bore passing through the cork, the warmth of the hand 



is quite sufficient to make the water rise very appreciably in the narrow 



tube. For though the glass expands, thus making the internal capacity 

 greater, the water expands still more, not only filling 

 up the additional volume of the flask, but also rising 

 in the tube. This is the principle which is used in 

 the ordinary thermometer, the bulb containing either 

 mercury or alcohol, and the expansion in excess of 

 that required to fill the increase of volume of the 

 bulb being indicated by the rise of liquid in the tube. 

 Gases expand still more than liquids. If a flask 

 partially filled with water be closed by a cork, 



. , through which passes a narrow tube with its lower 

 FIG. 2. Gauge, into , f. , . , . ., 



which a Bar fits enc ^ dipping under the surface oi the liquid, the 

 when cold but not warmth of the hand will make the air in the upper 

 when hot. portion of the flask expand and drive the liquid 



rapidly up the tube. Gas thermometers on this 

 principle may be made far more sensitive than the liquid thermometers, 

 but there are, as we shall see, difficulties in their use, which render them 

 unsuitable for ordinary purposes. 



The Construction of Mercury-in-Glass Thermometers. To 



construct a good thermometer for scientific purposes, a tube is selected 

 with a capillary bore as nearly uniform as possible. A bulb is blown on 

 the end of this, the size of the bulb being adjusted by the experience of the 

 glass-blower to the sensibility required in the thermometer the greater 

 the sensibility the larger the bulb or the finer the bore. In order to fill 

 the bulb with mercury, it is heated to expel some of the air, and the open 

 end of the tube is inserted under mercury. On cooling, the pressure of 

 the remaining air diminishes, and the external atmospheric pressure 

 drives some mercury up into the bulb. The tube is then held bulb 



