THE MEASUREMENT OF TEMPERATURE 



47 



easily seen. A scale, by means of which the changes may be 

 read, is either marked on the glass tube or firmly attached to it. 

 The scale is marked off in many small divisions called degrees, 

 which serve as units of measurement. 



47. Graduating the Thermometer. Before the thermom- 

 eter can be used, the scale must be adjusted to the height of 

 the liquid in the tube. In making this adjustment, two con- 

 venient fixed points from which to measure 

 are found in the temperature at which water 

 boils and the temperature at which it freezes. 

 After the liquid has been put into the tube, 

 the bulb is placed in melting ice which causes 

 the column of liquid to contract and shorten. 

 The point where the top of the column comes 

 to rest is marked freezing. Then the bulb is 

 placed in the steam from water boiling under 

 certain standard conditions until the liquid 

 column again comes to rest. This point is 

 marked boiling. The distance between the 

 two points is then marked off in degrees. 

 Cheap thermometers are made with less care 

 and their temperature points determined by 

 comparison with the scale of a more exact 

 instrument. The ordinary thermometer is ?] G - 13. Centi 



,, , . grade and Fahren- 



called a Fahrenheit thermometer after the heit thermometers, 

 man who invented it. On this thermometer, (Tower, Smith and 

 the scale begins at a point called zero (0) 

 which is 32 degrees below the freezing point. The boiling point 

 is marked 212, thus making 180 degrees between the freezing 

 and boiling points. A much better scale is that called the 

 Centigrade scale in which the freezing point is marked zero 

 and the boiling point 100. This is a decimal scale similar to 

 others already studied and is used almost universally for scien- 

 tific work. Its employment for other uses is steadily increas- 



