294 Thermal Conductivity and Emissivity. 



ition on rod 



Bridge 



-Reading in cm. 



in cm. 



Down. 



Up. Mean. 







73-8 



73'7 73-75 



5 



53-1 



52-6 52-85 



10 



38-0 



38-0 38-0 



15 



27-6 



27-5 27-55 



20 



19-8 



20-0 19-9 



25 



14-4 



14-7 14-55 



30 



10-6 



10-8 10-7 



35 



7-8 



7-8 7-8 



Temperature of air about 10^°. 



1 cm. of bridge-wire corresponds to ratlier less than one 

 degree. 



Dividing the above into two lots, and taking £ = '2386, we 

 get (this method is sufficiently accurate for this purpose) : — 



Mean temp. h. 



54° -000247 



42° -000240 



33° -000231 



27° -000229 



The mean temperature of the rod in the experiments by 

 Angstrom's method w T as about 27°, and the two emissivities 

 found were -000207 and '000242, so that the steady-state 

 emissivity comes just about halfway between the two. The 

 rod, however, had become somewhat tarnished during the 

 summer, and had to be cleaned again, which may or may not 

 have made some difference. Further experiments will be 

 needed to settle the point conclusively. 



When the last part of the experiments was in progress, 

 Mr. A. W. Porter, B.Sc, drew my attention to Messrs. Ayrton 

 and Kilgour's paper before the Royal Society on the variation 

 of emissivity with the diameter of the wires experimented 

 upon. The results obtained corroborate one another, and also 

 corroborate results that had been obtained some time ago by 

 Peclet, and which are recorded in his Traite de la Chaleur. 

 Peclet appears to have carried out a large number of experi- 

 ments on emissivity, the best account of which is found in the 

 3rd edition (not the 4th) in vol. i. and a long note at the end 

 of vol. iii. (published in 1860-61). 



In conclusion I must express my obligations to Prof. 

 Foster and Mr. Porter for much help given in the course of 

 these experiments. 



University College, London, 

 Dec. 29, 1894. 



