54 



FIRST YEAR SCIENCE 



currents were seen to be developed. The heated and ex- 

 panded water and air rose. Here the heat was transferred 



by the upward move- 

 ment of the heated 

 water and air. This 

 method of heat trans- 

 ference is known as con- 

 vection. The efficiency 

 of the hot water and hot 

 air furnaces which heat 

 our houses is due to the 

 convectional transfer- 

 ence of heat. We shall 

 find later that if it were 

 not for convection there 

 would be no winds or 

 ocean currents. 



If an incandescent 

 electric lamp is turned 

 on and the hand held 

 immediately below the 

 lamp, it will be warmed, 

 although the glass bulb 

 itself, a poor conductor 

 of heat, remains cool. 



Fig. 29. 



The white-hot filament is surrounded by an almost per- 

 fect vacuum. It can set up no convection currents, 

 neither does the cool glass. The sensation of heat can- 

 not be due to conduction because the air which surrounds 

 the bulb is not in contact with the filament. It is also a 

 poorer conductor than glass and the glass itself does not 

 become hot for some little time. 



There must therefore be another mode of transferring 

 heat beside conduction and convection. It also appears 



