MEASUREMENT OF TEMPERATURE 



79 



FlG - 63. Test-tube with 



water. 



the bulb is just out of the water and only heated by the steam. Again 

 record the temperature. In both cases the reading is the same. It is 

 either one hundred degrees (100), or very near it, if 

 you use a thermometer with Centigrade divisions. 

 (6) Repeat the experiment with a second lot of 

 pure water, and note that the temperature of 

 boiling water is again about 100. 



(c) Add salt to the water. Hold a thermo- 

 meter in the steam of the boiling water, and 

 notice that the temperature is the same as before, 

 namely 100. Push the thermometer into the 

 water, and notice that a higher degree of tem- 

 perature is indicated. 



(d) Again place the thermometer in clean ice 

 in a test-tube or flask. Gently heat the vessel, 

 and notice the following changes : 



(1) The mercury remains at until all the ice 

 is melted. 



(2) When the ice is melted the mercury rises 

 gradually until it reaches 100. 



(3) The mercury remains stationary at 100 

 until all the water is boiled away. 



iv. A thermometer cannot be deceived. 

 Arrange three basins of cold, lukewarm, and hot 

 water side by side. Place the thermometer in the 

 cold water and then in the lukewarm water. 

 Notice the temperature indicated in the lukewarm water. Now place the 

 thermometer in the hot water, and when it has been there a minute or two 

 put it into the lukewarm water. Notice that the temperature indicated 

 is practically the same as before. It is thus seen that, unlike the sense 

 of feeling, a" thermometer is not deceived by being made hot or cold before 

 using it to indicate temperature. 



v. The clinical thermometer. Examine a clinical thermometer. Notice 

 the large space between separate degrees. Observe that just above the 

 bulb the bore of the thermometer is constricted. Hold the bulb in the hand 

 and watch the expansion of the mercury. On exposing the thermometer 

 to the air and allowing it to cool, notice that the mercury thread breaks at 

 the constriction. The mercury can be shaken back into the bulb (Fig. 67). 



The fixed points on a thermometer. In the graduation of a 

 thermometer the plan usually adopted is to choose " two fixed points " 

 from which to number degrees of temperature. The most convenient 

 lower fixed point to select is the temperature at which ice melts, or 

 water freezes, for this is always the same if the ice is pure, and remains 

 the same so long as there is any ice left unmelted. Whenever the 

 thermometer is put into melting ice, the mercury in it always stands 

 at the same level ; or melting ice is always at the same temperature 

 and thus may be used to give one fixed point. The " higher fixed point " 

 chosen is that at which pure water boils at the sea-level. This stipu- 

 lation must be made, for the boiling point of a liquid is altered when 



