HIGH TEMPERATURE MEASUREMENT 507 



the quantity of heat radiation, T and Tq the absolute tempera- 

 tures of emitter and receiver, and k a constant, 



When T^ is atmospheric temperature and (7^— T^ exceeds say 

 500° C, 7"o* becomes practically negligible in comparison to 



Z*, and T becomes \l A- 



In order to make any radiation or optical pyrometer give 

 correct readings, it is necessary that the substance whose 

 temperature is to be measured should either be one by nature 

 approximately " black," like carbon, or that it should be placed 

 inside an enclosure in which it radiates under " black-body " 

 conditions. In practice the approximation to "blackness" of 

 many furnaces whose temperatures are to be measured is very 

 close. If it is undesirable to have an opening in the furnace, 

 a long tube closed at one end and made of iron, fireclay, or 

 other refractory material may be built into the furnace wall, and 

 readings are then taken by sighting on the bottom of this tube. 

 This arrangement, while giving a good approach to " black-body " 

 radiation, also serves to keep any flames which might be present 

 from exerting disturbing effects on the readings obtained. 



One of the most important of the new forms of instrument 

 is the Total Radiation Pyrometer of Eery, which depends for 

 its action on the law just enunciated. This pyrometer is none 

 other than the instrument used by the late Lord Rosse in 

 his researches on lunar radiation, modified to a smaller form 

 adapted for the purposes of temperature measurement. It con- 

 sists of a reflecting telescope of short focal length, which is 

 sighted upon the hot object. The radiation received is concen- 

 trated by a gilt concave mirror upon the junction of a very 

 minute and sensitive thermocouple placed at the focus. The 

 terminals of the couple are connected to a millivoltmeter, which 

 may be graduated to read directly the temperature of the body 

 on which the telescope is pointed. The usual types of the 

 instrument are graduated from 500° C. upwards and the sensi- 

 tiveness becomes greater the higher the temperature, differ- 

 ences of about 2° being measurable at the higher ranges. To 

 obtain this sensitiveness, however, the range of any one pattern 

 is usually limited to 700 or 800° from the first graduation. For 

 the millivoltmeter, if desired, a recording arrangement can be 



