THERMAL INTERCHANGE WITH ENVIRONMENT 



283 



Conduction 



The flow of heat through a medium with- 

 out the physical transfer of material is 

 called thermal conduction. Conduction, in 

 contrast to convection, can take place in 

 solids as well as in gases and Uquids, although 

 in the latter media appreciable conduction 

 occurs when the circumstances are such as 

 to prevent or limit convection. Heat is 

 conducted from within the human body to 

 the skin surface and from the skin into any 

 cooler objects with which the body may be in 

 contact. The heat lost from the normally 

 clothed body is probably largely conducted 

 through the thin air layers between the skin 

 and clothing. 



Laws of Thermal Conduction 



In the case of a medium with uniform 

 physical properties, it has been demon- 

 strated that the amount of heat which flows 

 from a warm surface to a cool one is directly 

 proportional to the length of the path, the 

 nature of the medium, and the thermal 

 gradient. Putting this in the form of an 

 equation, we have as the fundamental 

 equation for heat conduction in the steady 

 state : 



(7) 



Hn = 



KAjT, - T,) 

 d 



X t, gm. cal.. 



where Hd = quantity of heat conducted, 

 K = thermal conductivity, a constant 

 which depends upon the material, A = area 

 of the conducting surfaces, T2 and Ti = 

 temperature of the warm and cool surfaces, 

 t = time, and d = thickness of the con- 

 ductor. 



This formula has been applied by many 

 authors to the problem of the conduction of 

 heat from the interior of the body to the 

 skin. Herrington et at. (26) and Hardy and 

 Soderstrom (21) have shown that, when the 

 peripheral blood vessels of normal men are 

 fully constricted, between 9 and 10 kg. cal./ 

 wVhr./°C are conducted through the body 



tissues. This insulation corresponds to a 

 layer of tissue 18-22 mm. in thickness. 



The equation expressing the conduction 

 heat is entirely analogous to Ohm's Law for 

 electrical circuits, and it has been shown that 

 the conduction of heat through layers of 

 different materials can be expressed in 

 terms similar to those for electricity. In 

 the heat conduction equation, 



(8) 



Fo = 



{T, - T,) 

 _d_ 

 KA 



T2 — Ti is the thermal gradient and cor- 

 responds to voltage, Hd is the rate of heat 



flow corresponding to current, and ttt is the 



KA 



KA 



resistance factor, and —7- is the conductance 



factor. It is this factor which is important 

 when considering the insulation provided by 

 layers of clothing of different sorts. 



A further discussion of conduction as 

 applied to the problem of clothing is con- 

 tained in a later section concerned with the 

 use of an overall conductivity unit for cloth- 

 ing as introduced by Gagge, Burton, and 

 Bazett (16). 



To measure heat loss by conduction when 

 a steady state of heat flow has been at- 

 tained, four measurements are necessary: 



a. The skin temperature (Ts) over the 

 area of skin in contact with the con- 

 ducting medium, 



b. The skin area (A) in contact with the 

 conducting medium, 



c. The thiclmess (d) of the conductor, 



d. The temperature of the conducting 

 medium. 



The value of the specifi.c thermal con- 

 ductivity of the medium can probably be 

 found in one of the numerous physical 

 tables. 



Convection 



Convection is a term which refers to the 

 exchange of heat between hot and cold 



