The PiRELEss Cooker and Its Uses 1731 



principles used in modern cookers. At the World's Fair in Paris in 1867, 

 a Norwegian automatic cooker was exhibited and attracted fairly wide- 

 spread attention. It is within only about the last ten years, however, 

 that the fireless cooker has come to be a common piece of kitchen equip- 

 ment. In 1905 a United States consular report calling attention to 

 the use of the hay box in Germany led to an investigation of this method 

 of cooking by the commissary general of the War Department Present- 

 day scientific knowledge has made possible many modifications tending 

 toward efficiency and convenience. 



INSULATION 



Heat moves from one place to another place of lower temperature 

 by conduction, by convection, or by radiation. Conduction is the method 

 by which heat moves from one part of a substance to another without 

 visible motion of the parts of the substance; the heating of a flatiron 

 is accomplished by conduction. Convection is the method by which heat 

 moves from one place to another by the movement of the heated matter; 

 the heating of a room by a stove is accomplished in part by convection 

 currents in the air. Radiation is the method by which heat moves from 

 one place to another by means of ether waves; in tliis way the earth 

 receives heat from the sun. 



Certain substances are better conductors of heat than are others; 

 thus, metals are good conductors of heat, and nonmetals are poor con- 

 ductors of heat. This fact is well exemplified by the use of a fireless cooker. 

 First, the food is placed in a metal container, which readily conducts the 

 heat from the stove to the food; then the metal container is quickly passed 

 from the stove to its hole in the fireless cooker where it is surrounded 

 by some nonconducting material, which tends to prevent the escape of 

 the heat. Various nonconducting materials, or insulators, are used 

 for this purpose. 



The table (page 1732) from Carleton John Lynde's Physics of the House- 

 hold compares certain substances on the basis of the number of heat units 

 that pass through a layer of given thickness under given conditions. 

 Air is shown by this table to be the best nonconductor of heat, but 

 since it carries heat by convection currents, it would not be a good 

 insulator if used alone. The best results are obtained by using one of 

 the other substances of low-conducting power, packing it loosely enough 

 to contain air, but closely enough to prevent convection currents in the 

 air. The nonconducting property of the substances most often used is 

 due largely to the air that they contain. A vacuum is the most efficient 

 insulator, and it is used in the thermos bottle and in certain fireless 

 cookers. 



