1891.] The Thermal Emissivity of Thin Wires in Air. 169 



long thermometer which had been carefully tested between and 

 100 C., and the remainder of whose tube had been simply calibrated 

 for uniformity of bore. The consequence was that when we desired 

 to compare one of our thermometers reading, say. from 200 to 300 C C., 

 with the Kew standard, their bulbs were very far apart when both 

 were immersed in the oil-bath, and with the fops of the mercury 

 columns just above the surface of the oil. A short description is 

 given in the paper of the devices employed to overcome this difficulty 

 and which enable an accurate comparison to be made between the 

 thermometers. 



On examining the curves accompanying the complete paper which 

 show the emissivity for each temperature for each of the nine wires, 

 we see that : 



1. For any given temperature the emissivity is the higher the finer 

 the wire. 



2. For each wire the emissivity increases with the temperature, 

 and the rate of increase is the greater the finer the wire. For the 

 finest wire the rate of increase of emissivity with temperature is very 

 striking. 



3. Hence the effect of surface on the total loss of heat (by radiation 

 and convection) per second per square centimetre per 1 C. excess 

 temperature increases as the temperature rises. 



On comparing the loss of heat from the wire of 1*2 mils diameter 

 when at 300 C. with that from the wire of 6 mils diameter vrhen at 

 15 C., both being in an enclosure at 10 C., we see that the former 

 loses per square centimetre of surface per second not 



3-\ . or 58 times 

 15 10 



as much heat as the latter, as it would if the emissivity were ihv 

 same ; but, instead, 



60 x 58 or 3480 times 



as much heat ; arising from the fact that the emissivity, that is, the 

 number of calories (gramme C.) lost per second per square centimetre 

 of surface per 1 C. excess temperature of the 1'2-mil wire at 300 C., 

 is 60 times as great as that of the 6-mil wire at 15, the latter varying 

 very rapidly with the temperature near 15 C. 



From the curves the following table (p. 170) has been drawn up, 

 giving the emissivities of the various wires at eight useful tempera- 

 tures. 



We find that the emissivity of platinum wires of different diameters 

 at the same temperature can be very fairly expressed by a constant 

 plus a constant into the reciprocal of the diameter of the wire. For 

 example, we find that 



