311 



extraneous rays. The diapbragm E provides a constant angle aperture, 

 which is a necessary condition for the instrument to be independent of 

 focusing. The thermo-junction leads are connected to the posts b and bi, 

 which are in turn connected to a gahanometer. In malving a temperature 

 measurement the image of the incandescent object is focused upon the 



b b 



Fig. G. 



tliermo-couple by means of the eye piece E, and care must be fallen that 

 the image is larger than the thermo-couple. It is evident from Equa. 2 

 that if the galvanometer has a uniform scale and the temperature Ti is 

 liuown corresponding to a scale reading K,. the temperature T\ for any other 

 reading R, may be found from the relatio'n. 



When the limit of the scale is reached the calibration may be extended 

 by means of a diaphragm placed before the objective or by shunting the 

 galvanometer. In technical practice, however, a glass lens is used and 

 the instrument is calibrated empirically against a black body whose tem- 

 ]ierature can be determined. This instrument is also made with a gold 

 reflector instead of a lens. Fery'^ has recently brought out a new pyrome- 

 ter which is similar to the above with the exception that the tempera- 

 ture of the incident radiation is measured by means of a minute expansion 

 spiral consisting of two metals with dissimilar expansion coefficients. 

 This mechanical device renders the instrument more robust but does not 

 admit of so great accuracy as the theraioelement. 



Morse Thcrmo-Gage. This is somewhat similar to the Holborn- 

 Kurlbaum pyrometer in that it utilizes the disappearing filament principle 

 l)ut it is not nearly so precise since it is not providi^d with an.v lens system 



^ Engineering, May 14, 1909. 



