1887.] On Radiation from Dull and Bright Surfaces. 433 



XXIII. « On Radiation from Dull and Bright Surfaces." By 

 J. T. Bottomley, M.A., F.R.S.E. Communicated by 

 Sir W. Thomson, Knt., F.R.S. Received May 26, 1887. 



In connection with an investigation on heat radiation which I have 

 been carrying on for some time past, and on which I recently pre- 

 sented a communication to the Royal Society, I have had occasion to 

 examine the important results obtained by Mr. Mortimer Evans on 

 the radiation of light and heat from bright and dull surfaces when 

 incandescent (' Roy. Soc. Proc.,' vol. 40, 1886, p. 207) ; arid I have 

 repeated and verified some of his experiments. Mr. Evans experi- 

 mented on carbon filaments of incandescent lamps ; and in calculating, 

 for my own use, the resistances of the filaments at different degrees 

 of incandescence I was led to an unexpected result, and hence to an 

 investigation of which I desire just now to offer a preliminary notice. 



In order to explain, it is necessary for me to state briefly the object 

 and nature of Mr. Evans' experiments. Their object was the com- 

 parison of the radiation from surfaces having a bright, polished 

 appearance with that from dull surfaces having the appearance of 

 lampblack ; and he was led to an important practical conclusion as to 

 the superior light-giving efficiency of the brilliant-looking filament. 

 For these comparisons the same filament was treated in such ways as 

 to alter the surface from dull to bright and back again. It was taken 

 out of the glass globe for the purpose, and after treatment placed in 

 a fresh globe, which was then exhausted. The lamps thus constructed 

 and reconstructed were tested at various candle-powers, the energy 

 for each candle-power being determined. 



The tables given in Mr. Evans' paper show for the filaments in 

 different conditions the potential and the current required to maintain 

 different candle-powers from four candles upward. Using his numbers, 

 and supposing Ohm's* law to hold for the carbon filaments, I have 

 calculated the resistances of the filaments at different candle-powers. 



Two filaments used by Mr. Evans afforded satisfactory data for my 

 calculations. They are designated in his paper D, DD, DDD, and C, 

 CC, CCC. They had been treated in the following manner : — The 

 filaments D and C were "flashed " so as to have a dull surface with 

 the appearance of lampblack. DD and CC are the same filaments 

 flashed so as to have a brilliant surface, which, though black, has 

 something of the appearance of frosted silver. DDD, CCC, are the 

 same filaments again rendered dull as at first. The following table 

 shows the volts, amperes, and calculated resistances at the candle- 

 powers given in the left-hand column : — 



* I have already commenced an investigation into the question of the conformity 

 of carbon filaments at different temperatures with Ohm's law. 



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