186 Mr. J. T. Bottomley. Permanent Temperature [June 19, 



Experiments have been made by Mr. D. Macfarlane (" Proc. Roy. 

 Soc.," 1872, p. 93) for the purpose of determining the emissivity of 

 surfaces in absolute measure. Mr. J. P. Nichol has also obtained 

 results, which were communicated to the Royal Society of Edinburgh 

 by Professor Tait, 1869-70 ; and which have been reduced to absolute 

 measure by Professor Everett, " Units and Physical Constanta," 

 chap, ix, 137. 



On calculating the value of the emissivity for small copper wires 

 by means of the formula (4) given above, I have found in every 

 case a much greater emissivity than was obtained by these experi- 

 menters ; and so far as I have been able to go up to the present the 

 emissivity seems to increase as the diameters of the wires experi- 

 mented on are diminished. Macfarlane's determinations of emissivity 

 were made for a copper globe of 4 centims. diameter, and found to be 

 about 4 ,,'(, of the thermal unit C.G.S. per square centim. per second 

 per degree of difference of temperature for a polished surface with 

 a little over 60 of excess of temperature, and for a blackened 

 surface with excess of 5 or under. For round wires of small 

 diameter I have found very much larger emissivity than VCTTS- I 

 have obtained different values of e for wires of different sizes, varying 

 from -j^Vo with a polished wire diameter 0*83 millim., and excess of 

 temperature 27'5 C., down to -ffo with a wire of 0'40 millim., and 

 excess of temperature 24 C. 



In order to examine the question more thoroughly, I have com- 

 menced experiments in which I am endeavouring to do away the part 

 of the emissivity which is due to convection and carriage of the heat 

 by air. I have experimented to a certain extent on small wires in the 

 nearly perfect vacuum obtainable with the mercurial pump, and I 

 am preparing fof a more complete series of experiments. It seems 

 certain that this method of experimenting will give a very, accurate 

 way of determining the value of e in absolute measure for the 

 surfaces of wires. 



During the writing of this paper Professor Stokes has kindly 

 reminded me of the experiments made by Mr. Crookes (' Proc. Roy. 

 Soc.," vol. 31, p. 239). I find that my results, so far as they go, agree 

 perfectly with those of Mr. Crookes, showing a decrease in the 

 emissivity due to lowering the air pressure, this decrease being very 

 small for a reduction down to one-half or one-third of the ordinary 

 atmospheric pressure, but becoming very great with the almost 

 perfect vacuum obtained with the mercurial pump. 



The following table shows the emissivity of a copper wire with 

 bright surface half a metre long, 0*40 millim. in diameter, and sealed 

 iuto a glass tube, about 1*5 centim. internal diameter : 



