309 



The intensity of the unknown source in terms of tlie intensity of the 

 comparison lamp talcen as unity, becomes 



1 



J = (— )- 

 d 



\\liere d denotes tlie length of one side of the square aperture S. Due to 

 the lacli of monochromatism of the red ghiss tliis instrument is not so 

 accurate as the Wanner. 



IIolJ)orn-KurJl)auvi Pyvoinctcr. — In this method the huninous intensity 

 of the comparison source is varied until a photometric bahmce is obtained 

 between its image and the image of the incandescent object in question. 

 In tlie H.-K. (Holboni-Kurlbaum) pyronieter shown in Fig. 5 a small elec- 

 tric lamp L is placed in tlie focal plane of the objective O and the same 



Fig. 5. 



is viewed by means of an eye-lens E. In making an observation the pyrom- 

 eter is focused upon the object whose temperature is sought, thus bring- 

 ing the image of the object in the plane of L. The current through the 

 lamp is adjusted by means of a rheostat until the lamp filament disappears 

 against the bright background. The value of the current strength can be 

 read direct from a milli-ammefcer. 



In order to measure temperature with this instrument it must be 

 empirically calibrated by means of a black body. A curve may then be 

 plotted with current in milli-amperes, I, and temperature, t, in degrees C. 

 To determine an unknown temperature, it is only necessary to focus the 

 instrument upon the object in question and adjust the current through the 

 lamp until the filament disappears against the bright object. The pyro- 

 meter then indicates the black body temperature unless black body condi- 

 tions are realized, in which case it indicates true temperature, i. e., thermo- 

 dynamic temperature. 



The reading of the ;iimiK'ter will ])e independent i»f tlie distanci' of 

 pyrometer from (;bject si> long as the solid angle w, Fig. 5, is constant. 

 This is accomplished by means of the uiaphvagm D. When the instrument 



