CONDUCTION 95 



and water are used chiefly for transmitting heat by convection. 

 A furnace heats the water or air which in turn carries heat as it 

 travels throughout the building. 



Substances at the Same Temperature Feel Different. The 

 better heat conductors take heat from the hand more rapidly than 

 do poor conductors. In the same room, with all objects 

 at the same temperature, the bare floor will feel colder 1 1 i 1 1 

 than a rug. This is because the floor takes the heat 

 away from the hand more rapidly than the rug. 



Place a thermometer on several objects in the room which 

 seem of different temperatures. The thermometer will register 

 the same temperature for all. 



With the hand, test as many substances in the room as 

 possible. Tell whether they are poor, medium, or good con- 

 ductors. 



Different Substances Conduct Heat at Different Rates. 

 Cover several pieces of wire of different metals with heat- FIG. 73. 

 indicating paint. Arrange each metal so that one end is over 

 the mouth of a flask from which steam is coming. The other ends of the 

 metals may rest on blocks of wood. Prove that the metals conduct heat 

 at different rates. Compare the rates you obtain with the table on page 96. 



Heat-indicating Paint. Equal parts of 'iodide of mercury and iodide of 

 copper rubbed down with sufficient distilled water to produce a spreadable 

 paste make a temperature-indicating paint which turns black between 140 

 F. and 206 F. and returns to its red color on cooling. (Red iodide of mer- 

 cury works best.) 



A yellow iodide of mercury and of silver is far more sensitive to heat than 

 the above paint. The yellow paint changes from yellow to red as the heat 

 is conducted along the metal. 



From the table it will be seen that silver is the best conductor of 

 heat and air one of the poorest. 



Try several teakettles, placing the same amount of water in 

 each and noting the time required to bring the water to boiling 

 point. If the kettles do not have bottoms of the same size deter- 

 mine whether this has any effect on the time required to heat the 

 water, and the consequent waste or saving of gas. Try to find other 

 important factors beside the conductivity of the metals. Measure 

 the amount of gas consumed, and determine the amount saved if 

 kettles of the proper kind are used. 



