Thermal Radiation in Ahsoktte Measure. 



275 



passing and resistance. The consequence is that owing to the con- 

 stantly altering state of the vacuum an irregularity is introduced 

 in the loss of heat, and the irregularity tells more in the case of small 

 wires than in the case of larger sizes. 



In the case of the bright wire, Pts, the loss of heat was somewhat 

 abnormal. It is probable that the surface was lacking in polish. 



It will be seen that the loss from the sooted platinum wires is about 

 four to five times that from the bright wires at the same temperature. 

 In the paper of 1894, already referred to, the radiation from a very 

 brightly polished and burnished silvered copper globe, and that from 

 the same globe sooted, were determined. The highest temperature 

 reached was about 230" C, and in that case the sooted globe lost about 

 ten times as much heat as the silvered globe under the same circum- 

 stances. When the silvered globe had become tarnished, the radiation 

 from its surface was so much increased that the loss from the sooted 

 globe was only three times that from the tarnished silver. 



Table I.— May 18, 1897. Two Platinum Wires, Pti and Pts, length 

 42*55 cm., diameter 0*0542 cm., from the same hank of wire. Pti 

 left with bright surface, Pto thinly sooted. 



OS 



O 



Pti. 



Pt.. 



Pressure in millimetres. 



Current in amper 



Resistance in 

 ohms. 



Temperature. 



Thermal energy 

 lost per square 

 centimetre per 

 second. 



1 



Resistance in 

 ohms. 



Temperature. 



Thermal energy 

 lost per square 

 centimetre per 

 second. 



1 



0-023 



0-192 



17 



033 X 10-* 



0-188 



17 



-0329 X 10-" 



-00025 



0-276 



0-208 



33 



5-198 



0-198 



26 



4 -952 



-00025 



0-552 



0-259 



89 



25-98 



0-209 



36 



20 -96 



-00045 



0-695 



0-292 



130 



46 -52 



0-220 



51 



35-02 



-00045 



0-940 



0-337 



185 



97-97 



0-237 



71 



68-90 



-00045 



1-430 



0-415 



295 



279-80 



-271 



109 



183-1 



-00045 



1-937 



0-484 



453 



599-5 



0-317 



167 



393 



-00060 



2-691 



0-570 



623 



1371 



0-377 



249 



912-2 



-00050 



3-003 



0-599 



743 



1776 



0-398 



280 



1184 -0 



-00060 



3-770 









0-437 



343 



2055 -0 



-00500* 



4-446 









0-476 



414 



3106 -0 



-00360 



5-200 









0-515 



496 



4517 -0 



-00320 



6-604 









0-566 



643 



8166-0 



-00250 



* Owing to increase in the pressure, the emission here must be considerably 

 increased by convection. 



