1884.] Energy and Radiation in Incandescence Lamps. 165 



potential was read by one of Siemens' volt-meters. In every case the 

 potential was taken close to the lamp. Grove's cells were used to 

 obtain the necessary currents. 



The unbroken lines in fig. 2, as we have already said, show the 

 forms of the curves at different parts of the spectrum. The light 

 used in this experiment was from one of the British Electric Light 

 Company's lamps. 



The fiducial energies measured were ISO'S, 130'S, ]16'4, 93'8, and 

 33' 6 watts. From these the hyperbolas were calculated. Table IX 

 gives the calculated and observed deflections, from which figs. 3 and 4 

 were constructed, and also the total radiation as observed on the 

 thermopile. The observed and calculated radiations for different 

 parts of the spectrum agree very closely. 



The wave-lengths corresponding to the position of the thermopile 

 at different turns of the screw in this case were as follows : 



Table X. 



Turns of screw. X 



5,900 



1 6,150 



2 6,450 



3 6,700 



4 7,050 



5 7,400 



6 7,800 



7 8,250 



8 8,800 



9 9,500 



10 10,300 



11 11,200 



12 12,200 



m 12,700 



Turns of screw. X 



13 13,200 



14 14,300 



15 15,500 



16 16,700 



17 17,900 



18 19,150 



19 20,400 



20 21,700 



21 23,000 



22 24,300 



23 25,700 



24 27,100 



25 28,600 



These, as well as the wave-lengths shown in the other tables, were 

 obtained by taking photographs of substances giving lines which 

 have previously been mapped. The back of the pile was covered 

 with thick felt, and kept at a constant temperature by means of ice. 

 A screen was placed in front of the slit. No radiation hence fell on 

 the apparatus except for the instant required to cause the needle to 

 swing. 



In fig. 2 the dotted line shows the results obtained by measuring 

 the total energy of radiation from the lamp. It will be seen by a 

 reference to our paper (" Proc. Roy. Soc.," vol. 35, p. 328) of 1883, 

 that the energy of radiation for the visible spectrum of incande- 



