216 WATER, HEAT, AIR, AND LIGHT IN THE HOUSE 



the bar is heated, the unequal expansion of the two metals 

 will produce a curved bar. If we fasten one end of the 

 compound bar, leaving the other end free, the free end 

 will be able to push against a pointer, and so to record the 

 temperature. 



In the thermostat (Fig. 204), an instrument used to 

 regulate the temperature of a room, the bar is made a 

 part of two electric circuits; it will permit 

 the current to flow in one circuit when the 

 room becomes too warm, and through the 

 other circuit when the room becomes too 

 cold. Electromagnets in the two circuits 

 control the supply of steam, hot water, etc., 

 turning it on or off as needed. 



245. Exercises. 



1. What is the source of the water in the city in 

 which you live? Is the water hard or soft? Is it 

 purified in any way before it enters the mains? How 

 large are the mains? Is your city water ever tested 

 for impurities? What have been the results of the 

 tests for the last two months? How much does your 

 family pay for water? How many cubic feet of water 

 does your family use in a month or in a quarter? 

 2. What is the form of the plumbing " traps" in your house? Are 

 they like the trap shown in Fig. 200? If your basement or barn has a 

 floor drain, find out the form of the trap, and describe it. 



3. How are the outside faucets of your house emptied to prevent 

 the water in them from freezing in winter? Why not let the water 

 freeze in them? 



4. Suggest some ways of kindling fire without the use of friction. 



5. Show that the wood and coal cook stove is really an improved 

 form of fireplace. Show that the fireplace fire is an improved form of 

 the open fire built against a rock. 



Fig. 204. 



Thermostat. I f 

 the room be- 

 comes too warm, 

 the brass ex- 

 pands more than 

 the iron, and 

 turns the rod to 

 the right ; so that 

 it makes a circuit 

 which cuts off 

 the heat. 



