HEAT. 



decreased. The index above a is left in its initial position, while the 

 index above b is pushed to the furthest position reached by the mercury. 

 If, on the other hand, the temperature falls, the liquid in G contracts, the 

 point a rises and pushes its index in front of it, while the index above b 

 is left in its initial position. Two graduated scales are fixed behind the 

 two limbs of the U, that on the left running from above downwards, 

 that on the right from below upwards. Evidently the former gives 

 minimum and the latter maximum temperatures. The instrument is 



Nonconducting Ltd 



Manometer 

 To Air Pump 



FlG. 7. Constant-Temperature Apparatus, using the Boiling -Point Method. 



reset after an observation by moving the indices back to the ends of the 

 mercury thread by the magnet. 



The liquid in G is here the chief expanding liquid. The mercury is 

 little more than a device for moving the indices. 



Thermostats, or Constant-Temperature Instruments. In a 



great number of physical experiments, it is desirable to keep a body at a 

 known fixed temperature for a considerable time. There are certain 

 temperatures which are easily maintained, as, for instance, that of melt- 

 ing ice. A body placed in melting ice, or in a chamber surrounded by 

 melting ice, will remain indefinitely at C, if proper precautions are 

 taken to keep the temperature of the ice and water uniform either by 

 stirring or by surrounding the vessel with a second vessel containing a 

 similar mixture. There are also definite " freezing mixtures " which 

 give fairly constant temperatures below 0. Or, again, the steam from 

 water boiling in a metal vessel is very nearly 100 at ordinary altitudes, 



