86 



Our Surroundings 



How Temperature Is Measured. Temperature is most com- 

 monly measured by a thermometer. A standard form consists of 

 a tube with a very small bore, called a capillary tube, with a bulb 

 at the lower end. This tube is partly filled with mercury. The 

 air is exhausted from the part above the mer- 

 cury. The tube is sealed at the upper end and 

 fastened on a metal or a wooden frame, with 

 a scale indicating the different degrees of tem- 

 perature. The action of this instrument de- 

 pends on the expansion of the mercury as it 

 warms and on its contraction as it cools. 



Some thermometers contain colored alcohol 

 in place of mercury. Others, used in measur- 

 ing very high temperatures, are made of bars 

 of copper or some other metal, whose expan- 

 sion or contraction is recorded on a dial. These 

 metal thermometers are often used on ovens. 

 Two thermometer scales are in common 

 use. In this country we use for ordinary pur- 

 poses the Fahrenheit scale. It has two fixed 

 points : the boiling point of water, which is 

 marked at 212 degrees, and the freezing point 

 of water at 32 degrees. The space between 

 these two points is divided into 180 degrees. 

 Scales usually read far below 32 degrees. 

 When the temperature goes below 0, we say 

 it is so many degrees below zero, and use a 

 minus sign to express it. Eight degrees below 

 zero is written 8. 



In Europe the centigrade thermometer is 

 quite generally used. On its scale the boiling 

 temperature of water is placed at 100 degrees 

 and the freezing temperature at degrees. 

 The space between these points is divided into 

 100 degrees. Temperatures below zero are marked minus ( ). 

 The centigrade thermometer is used in this country for most 

 scientific purposes. 



THERMOMETER 

 SCALES COMPARED 



The scale marked C 

 is centigrade. The 

 scale marked F is 

 Fahrenheit. Compare 

 the freezing and boil- 

 ing points on the two 

 scales. 



