MEASUREMENT OF TEMPERATURE 81 



liquid, or metal thermometers. There must be some 

 uniform temperatures between which the expansion shall 

 be measured if we are to have a basis of comparison. These 

 definite points have been taken as the freezing and boiling 

 points of water at sea level. 



Experiment 28. (Teacher's Experiment.) Fill a four-inch 

 ignition tube with mercury and insert a one-hole rubber stopper 

 having a straight glass tube extending through it and about 20 

 cm, above it. (Figure 36.) It may be necessary 

 to cover the stopper with vaseline to keep out 

 air bubbles. When the stopper was inserted the 

 mercury should have risen a few centimeters in 

 the tube. Mark with a rubber band the end of 

 the mercury column. Gently warm the ignition 

 tube. The mercury column rises. Gool the 

 tube and the column falls. We have here a 

 crude thermometer. 



The substance whose expansion is most 

 commonly used to measure the degree of 

 temperature is mercury. This expands 

 noticeably for an increase in temperature FIGURE 36 

 and the amount of its expansion can be very 

 readily determined. The ordinary thermometer consists of 

 a glass tube of uniform bore which has a bulb at one end. 

 The bulb and part of the tube are filled with mercury. The 

 remaining part of the tube is empty, so that the mercury 

 can freely rise or fall. When the temperature rises, the 

 mercury expands and rises, when the temperature falls, the 

 mercury contracts and sinks. 



There are two kinds of thermometer scales commonly 

 used. The one which is used almost exclusively in scientific 

 work and in those countries where the metric system of 

 weights and measures has been adopted, is called the Cen- 



