307 



the measurement of temperature is made yhotometrirally by adjusting to 

 equality two photometric fields produced by a standard source and the 

 body to be measured. The intensity of radiation is varied by cutting down 

 the objective aperture, as in Le Ohatelier, or by a polarizing device, 

 as in the Wanner, or by varying the iutensitj'' of the standard itself, as in 

 (he Holborn. 



Since we are using mono-chromatic light a measure of the luminous 

 intensity may be taken as a measure of the radiant energy. The intensity 

 of radiation of a source may be defined as the ratio of the total energy 

 ejiiitted (including all wave lengths) to the energy falling upon unit sur- 

 face. A part of the energy emitted by a heated body, however, may be 

 luminous and both the luminous and total energy emitted by a body in- 

 creases with temperature, but the total luminous energy is not proportional 

 to the total energy emitted. The luminous energy of any particular wave 

 length, however, is directly proportional to the total radiant energy 

 emitted. Hence in any optical pyrometer when photometric comparison 

 is made if mono-chromatic light is used the above radiation laws will 

 hold. 



Wanner Pyrometer. — It has been shown that the luminous intensities 

 of two bodies maj be taken as a measure of their temperatures, if mono- 

 chromatic light is used, and since luminous intensities may be compared 

 by the rotation of a Nicol prism we have a convenient means of measuring 

 high temperatures. 



In this method comparison is made between a standard lamp and the 

 body whose temperature is sought. The standard used is a 6-volt incan- 

 descent lamp which is in turn compared with some primary standard as 

 an amyl acetate lamp. For this work the primary standard is used merely 

 as a check for the more convenient electric lamp and so long as it is re- 

 producible so that the comparison lamp can always be brought to the same 

 condition, we are not concerned with its intrinsic intensity or temperature. 

 Photometric comparison is made of the comparison lamp and the unknown 

 source by adjusting to equal brightness two halves of a photometric field 

 by means of a polarizing arrangement, monochromatic red light being pro- 

 duced by a direct vision prism. 



The intensity of llic niil<n()wn source in terms of tlie comparison lamp, 

 taken as unity, is 



- J = tan^0 

 where w, is the rotation of the Nicol prism. 



