1891.] The Thermal Emissivity of Thin Wires in Air. 167 



emissivity that is produced by change in the shape and size of the 

 cooling body. Indeed, so little has been the attention devoted to the 

 very large change thab can be brought about in the value of the 

 emissivity by simply changing the dimensions of the cooling body, 

 that in Professor Everett's very valuable book of Units and Physical 

 Constants the absolute results obtained by Mr. Macfarlane are given 

 as the ; ' results of experiments on the loss of heat from blackened and 

 polished copper in air at atmospheric pressure," and no reference is 

 made either to the shape or to the size of the cooling body. 



[November 19, 1891. Since this paper was sent in to the Royal 

 Society, a new edition of this book has appeared, and, in consequence 

 of a suggestion made to Professor Everett, the word " balls " has been 

 added after the word " copper " in this new edition, as well as the 

 following paragraph : 



" Influence of Size. 



" According to Professor Ayrton, who quotes a table in ' Box on 

 Heat,' the coefficient of emission increases as the size of the emitting 

 body diminishes, and for a blackened sphere of radius r cm. may be 

 stated as 



T 



" The value in M'Farlane's experiments was 2."] 



The laws which govern the loss of heat from thin cylindrical con- 

 ductors have not only considerable scientific interest in showing how 

 the shape of a body affects the convection currents, but they are of 

 especial importance to the electrical engineer in connexion with glow 

 lamps, hot-wire voltmeters, fuses, &c. We, therefore, thought it 

 desirable to ascertain the way in which the law of cooling for thick 

 wires, which involved the diameter raised to the power three halves, 

 passed into the law for the cooling of thin wires, involving only the 

 first power of the diameter. For this object, the investigation described 

 in the paper was commenced at the beginning of 1888, and the 

 emissivity was measured of nine platinum wires, having the diameters 

 of 1-2, 2-0, 2'9, 4-0, 6-0, 8'1, 9'3, 11-1, and 14 mils, or thousandths of 

 an inch. 



Suspecting that some of the published results concerning the currents 

 required to fuse wires had been much influenced by the cooling action 

 of the blocks to which the ends of the wires were attached, we started 

 by making a calculation of the length necessary to give to our wires, 

 so that the loss of heat by conduction should not introduce any im- 

 portant error into the determination of the emissivity. To do this it 

 was necessary to calculate the distribution of temperature along a 



N 2 



