232 



Mr. W. Makower on a .Determination of the 



no time been allowed for the air round the thermometer to 

 heat up. For since 



Sl)-*^**©' • • • • ;< 2) 



we see, by expanding by the logarithmic series and neglecting 

 all terms except the first, that 



(0 X _ e,) = eP~~ p2 ?—± . . (approximately) (3) 

 Pa 7 1 



y — 1 7 — *- 



Hence *■ is proportional to the fall of temperature. — — 



as calculated from (2) was plotted against t, and by extra- 

 polating back to t=0 the value of corresponding to 



no heating of the thermometer, due to conduction and con- 

 vection, was obtained. 



In Table II. are given the values obtained : — 



Table II. 



Time of closiug 





y-1 



circuit in seconds 



7- 



(0- 





7 



0-7G 



1-396 



•2837 



112 



1-396 



•2837 



]-90 



1-386 



•2785 



3-33 



1-381 



•2759 



1-83 



1-386 



•2785 



1-45 



1-392 



•2816 



3-00 



1-380 



•2754 ' 



2-30 



1-386 



•2785 



5-00 



1-380 



•2754 



4 01 



1-380 



•2754 



095 



1-396 



•2837 



5' 15 



1-380 



•2754 



1-90 



1-389 



•2801 



1-65 



1-389 



•2801 



213 



1-391 



•2811 



2-15 



1-391 



•2811 



7 — 1 

 Assuming the variation of — with t to be linear over 



the small range considered, the value of ' corresponding 



tot = is-285. Hence r =l-399. ? 



(2) The error due to radiation was allowed for by coating 

 the thermometer with platinum black. Assuming that the 

 absorption by a platinum-blacked surface is 15 times as 

 powerful as that of a bright surface*, the error due to 

 radiation could be estimated. 



* Lummei' mid Pringsheini. he. cit. 



