278 Mr. A. W. Porter on the Influence of the 



in which .<e=10 centim., and the value of k taken is that 

 determined by Mr. Eumorfopoulos for the same rod by 

 Angstrom's method, viz. '2386. The emissivity is there- 

 fore in each case an average value for the range of temperature 

 between the two points. The actual temperatures were 

 ascertained by standardizing the thermoelectric couples in 

 water-baths of known temperatures. 



The above results are also shown plotted in fig. 3. An 



Fig. 3. 



Variation of Emissivity of Cylindrical Brass Rod with the Radius of its 



Cylindrical Enclosure. 



Radius of rod = -483 centini. 

 •0002 



\> 



'S3 



CD 



S 

 W 



•0001 



10 15 



Radius of enclosure in centim. 



elastic curve (bent steel lath) has been made to pass through 

 the two results for definite dimensions and extended pro- 

 visionally by a dotted line so as to rise asymptotically to the 

 highest value. It is unsafe to rest too much upon these few 

 results : more will need to be obtained before the law of 

 variation can be stated with certainty : the following paragraph 

 must be considered therefore as merely provisional. Take the 

 two definite values and also (by interpolation) the value for 

 an enclosure of 2' 54 centim. radius (that employed by Messrs. 

 Ayrton and Kilgour) and find the values of " b" They 

 are : — 



Radius of 



enclosure. 



R. 



Radius of rod. 

 a. 



b. 



1:6 



254 

 515 



•483 

 •483 

 •483 



•029 xlO" 3 



•0418 „ 

 •0643 „ 



The value derived from A. and K/s results at 60° is 



