Example.—To compare the energy radiated through an opening in the side of a furnace at temperatures of 527°, 727°, and 927° C. respectively, to surroundings at 27° C.
The quantities will be as
K (8004 - 3004) : K (10004 - 3004) : K (12004 - 3004).
since 273 must be added to each temperature to convert into absolute degrees. Dividing each by K, and expanding in each case, the ratio becomes
(4096 - 81) × 108 : (10000 - 81) × 108 : (20736 - 81) × 108.
Dividing each by 108 and subtracting, the result is
4015 : 9919 : 20655, or 1 : 2·47 : 5·12.
It will be noted in the above example that the effect of the surrounding temperature, taken as 27° C., is small in quantity, and becomes proportionately less as the temperature of the furnace increases. If T2 had been ignored in the calculation, the amounts of energy radiated would have appeared as
1 : 2·44 : 5·06.
It will be seen later, that in calculating the temperature scale of a radiation pyrometer, the temperature of the surroundings is for this reason not taken into account. [Fig. 43] is a graphic illustration of the fourth-power law.