178 ILLINOIS ACADEMY OF SCIENCE 



forced to base our calculation on the air temperatures on the 

 two sides of the wall since in any problem of this sort we are 

 only concerned with maintaining some fixed or desired internal 

 "room" temperature when a given "outside" temperature 



exists. 



3. A reference to Figure 1 will show that these two air 

 temperatures t and t respectively, are not the same as the cor- 

 responding surface temperatures, indicated by tj (inside) and 

 t 2 (outside), due to the fact that the surface film of air pro- 

 tects or jackets the surfaces in such a way that there is always 

 a drop in temperature right at the surface as shown by the 

 temperature gradient line. Moreover, this drop in tempera- 

 ture t-tj, or t 2 -t varies with the character of the wall material 

 and the rate of air movement over the surface. It is, of course, 

 possible to calculate the heat transmission through the wall 

 from the conductivity of the wall material alone provided the 

 surface temperatures are given. The heat H transmitted by 

 conduction would be, in B. t. u. per sq. ft. per hour, (tilt 2 ) 

 where C= the conductivity in B. t. u. per sq. ft. per unit thick- 

 ness, and x= thickness in the same units; but the two terms 

 of the binomial are unknown. In other words we must employ 

 a unit of transmission based on air temperatures rather than 

 surface temperatures. 



4. If we now consider the phenomena taking place at either 

 surface of the wall, we find that heat is being transferred (in 

 the case of the inner surface) from the air and objects within 

 the room, which are considered to be at the same temperature 

 as the air; (1) by radiation from these objects; (2) by con- 

 vection currents moving over the face of the wall and thereby 

 losing heat by contact, as shown in Figure 1. Unfortunately, 

 the determination of separate coefficients for radiation and 

 convection for various wall surfaces is a most difficult prob- 

 lem ; whereas the experimental determination of a single com- 

 bined coefficient is a fairly simple matter. Moreover, the use 

 of such a composite coefficient materially simplifies the calcu- 

 lation of suitable units of transmission based on air tempera- 

 tures alone. 



5. Assuming this combined surface coefficient K is known, 

 we can find another expression for the heat H transmitted per 

 sq. ft. of wall surface, which is K x (t-tj) for the inside, and 



