Ne 
ry 
— 
extraneous rays. The diaphragm I) 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 hh, 
which are in turn connected to a galvanometer. In making a temperature 
measurement the image of the incandescent object is focused upon the 
thermo-couple by means of the eye piece E, and care must be taken 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 T: is 
known corresponding to a scale readiug R,, the temperature T, for any other 
reading R, may be found from the relation, 
P,=T. VR, 
When the limit of the scale is reached the calibration may be extended 
by means of a diaphragm placed before the objective er 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- 
perature can be determined. This instrument is also made with a gold 
reflector instead of a lens. Féry’ 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 thermoelement. 
Morse Thermo-Gage. This is somewhat similar to the Holborn- 
Kurlbaum pyrometer in that it utilizes the disappearing filament principle 
but it is not nearly so precise since it is not provided with any lens system 
7 Engineering, May 14, 1909. 
