128 



THE PRESSURE DUE TO RADIATION. 



to Caiv 



toGalv. 



THE ENERGY MEASUREMENTS. 



The radiant intensity of the beam used in the experiments was deter- 

 mined l)y directing- it upon the blackened face of a silver disk, weigh- 

 ino- -t.SO grams, of 13.3 mm. diameter, and of 3. 58 mm. thickness, and 

 by measuring its rate of temperature rise as it passed through the 

 temperature of its surroundings. The disk was obtained from Messrs. 

 Tiffany & Co., and was said by them to l)e 99.8 per cent line silver. 

 Two holes were bored through parallel diameters of the disk, one- 

 fourth of the thickness of the disk fi'om either face. Two iron con- 

 stantan thermojunctions. made l)y soldering 0.1 mm. wires of the two 



metals, were drawn through 

 the holes into the center of the 

 disk. To insulate the wires 

 from the disk, tine drawn- 

 glass tubes were slipped 

 over them and thrust into 

 the holes, leaving less than 2 

 nun. bare wire on either side 

 of the junctions. The wires 

 were sealed into the tubes 

 and the tubes into the disk 

 by solid shellac. The tubes 

 projected 15 mm. oi* more 

 from the disk and were bent 

 upward in planes parallel to 

 the faces of the disk. The 

 general arrangement will lie 

 seen in tig. 5. The disk was 

 suspended l)y the four wires 

 some distance below a small 

 flat wooden box. On the l)ox 

 was fastened a calorimeter 

 can swathed in cotton and 

 tilled with kerosene in which 



'' '' ''' the constant thermojunc- 



FiG. 5. . . ^ \^ 



tions were nnmersed. Cop- 

 per wires soldered to the two ends of the thermoelectric series were 

 brought out of the calorimeter, and the circuit was closed through 

 1,000 ohms in series with the 500 ohms resistance of galvanometer Gj. 

 The thermojunctions in the disk were in series, and as each junction 

 was midway betw^een the central plane of the disk and either face, it 

 was assumed that when the disk was slowly warmed by heating one 

 face the electromotive forces obtained corresponded to the mean tem- 

 perature of the disk. One face of the disk was blackened by spray- 

 ing it with powdered lampblack in alcohol containing a trace of shellac. 



Airspace. 



