i;»i2] 



on Very High Temperatures. 



319 



Some of the nietliods for measuring temperature with their 

 limitations are briefl}' recapitulated in Table i). I have only time to 

 refer to one or two points. We have recently had tlie opportunity at 

 the National Physical Laboratory of subjecting a number of mercury 

 in silica thermometers to a critical examination. These thermometers 

 which are made in England, possess in a high degree the qualities of 

 constancy, large range, and such complete freedom for temporary zero 

 change that I feel safe in prophesying they Mall inevitably replace the 

 present International standards, which are made of verre dur. 



Table 3, 



-Some Indication op the Present Range of Temperature 

 IMeasdring Instruments. 



Expansion thermometers — 

 Gas thermometer 

 Mercury in glass 

 Mercury in silica 



Electrical tJiermometers — 

 Platinum resistance 

 Thermocouples : 



— platinum alloys 



— base metals 

 Total radiation pyrometen 

 Optical pyrometers 



600° ,, 3500° No upper limit 



In regard to high temperatures most of us rely to some extent on 

 colour in estimating temperature. Table 4 gives a very fair notion 

 of the temperature we may reasonably associate with the colour of a. 



Table 4. — Temperature and Colour op a Fire. 



" Grey," lowest discernible temp. 



Very dull red 



Dull red . . . 



Cherry red . 



Orange 



White .... 



Dazzling white 



About 450° 



500° 



700° 



900° 



„ 1100° 



„ 1300° 



Above 1500° 



About 850° 

 950° 

 „ 1300° 

 „ 1650 

 „ 2000° 

 „ 2400° 



Above 2750° 



fire or muffle furnace (experiment shown). The intensity of the 

 ligut varies according to well-known laws which have been studied 



