THE REFRIGERATOR 



155 



several thicknesses, some of the materials being selected be- 

 cause they are poor conductors of heat. Because air is a 

 poor conductor of heat, most refrigerators are constructed 



with an air space in the walls. This air . 



space is usually packed somewhat loosely with 

 some substance such as charcoal or mineral 

 wool or other poor conductors. This packing 

 serves to break up convection currents which 

 otherwise would be produced in this air space 

 (Fig. 117). Air is a poor conductor of heat 

 when not in motion, but, as we saw in the 

 study of furnaces, it is a very effective agent 

 in the transference of heat when it is moving 

 in the form of convection currents. 



184. Styles of Refrigerators. Refrigerators are either 



TOP-ICING KEFRIGERATORS (Fig. 118), Or SIDE-ICING REFRIG- 

 ERATORS (Fig. 119). In the top-icing refrigerator the cold, 

 descending column of air may pass downward at the center 



FIG. 117. Walls 

 of a refrigerator. 



FIG. 118. Top-icing refrigerator. 



FIG. 119. Side-icing refrigerator. 



of the refrigerator while the warmer, lighter air passes upward 

 at the two sides as shown in Fig. 118; or the cold, descending 

 column may pass downward at one side of the refrigerator 



