138 W. Ferret — Weber's Law of Thermal Radiation, 



required. We come now to a similar examination of Weber r s- 

 new law.* 



2. Putting s for the rate with which energy is emitted in all 

 directions by the rays of wave-length /t from the surface F of 

 the radiating body at the absolute temperature T, he assumes 



s=C7tF—.e b*T*\" K } 



in which ^='0043 and b and c are constants which vary with 

 different substances. This makes s increase with the increase 

 of T more rapidly for the rays of short than of long wave- 

 lengths, as it should, and is found to represent approximately 

 the experimental results obtained by Langley. But experi- 

 ments on the radiation of the separate wave-lengths are too few 

 yet to furnish data for an accurate test of this formula. 



3. Putting now S for the rate with which heat of all wave- 

 lengths is radiated, he gets from the preceding expression of s 

 by integration, . 



S=f™sd\ = CFT. e aT . (2) 



This gives for the rate with which the surface F of a body at 

 absolute temperature T loses heat when placed in an inclosure 

 of absolute temperature T x , in Weber's notation, 



^St,t 1 = CFT 1 .e aTl | ^. e a ^-^)-\ j . (3) 



The value of C in these expressions is a function of b and c in 

 (1) above, and therefore is different in different substances. 



The values of the first member of this expression being 

 obtained from experiment with an inclosure of absolute tem- 

 perature T, for a number of values of T extending through a 

 considerable range, the test of the formula is that we must have 



T 



= CFT, .e aT >= a constant. 



rTT.^-^-l W 



4. In testing the formula Weber used the results of Schleier- 

 macher's experiments,f in which the radiation was determined 

 by the amount of electric energy consumed in maintaining a 

 platinum wire at a given temperature, stretched in vacuo in 

 the interior of a tube. In one of the sets of experiments a 

 bright wire, 0*197 mm in diameter, was used, and in the other a 

 wire coated with the black oxide of copper, 0'296 mm in diame- 



* Sitzungsb. der Kon. Preuss. Ak. der Wissenschaften, 1888 (2), 933. 

 f Wied. Annalen, xxvi, 287. 



