ly 
whose temperature can be varied. This is generally done by means of an 
electric furnace, but when a Wanner, Le Chatelier, or a calibrated H.-Is. 
is at hand it is easiest accomplished by direct comparison. 
The comparison source may be a thin platinum strip, heated electric- 
ally or a wide filament incandescent lamp. The H.-IKk. may be sighted on 
one side of the strip and the calibrated pyrometer on the opposite side. 
The black body temperature of the strip can be determined by means of the 
calibrated instrument and at the same time the reading of the H.-K. com- 
parison lamp can be taken. 
In the case of a wide filament carbon incandescent lamp it has been 
shown that if it is properly aged for about 20 hours at 1,760° C. it will 
remain sufficiently permanent for a secondary standard for 15 or 20 hours. 
If a lamp is calibrated in terms of black body temperature and current 
strength by means of a pyrometer it may be used as a standard of compari- 
son for calibrating pyrometers just as a black body would be used. 
CALIBRATION. 
In the foregoing a number of instruments have been described for the 
estimation of high temperature, each class utilizing some effect of tempera- 
ture such as the change of resistance, development of small electromotive 
force, change of luminous intensity, etc., and it now remains to indicate 
how each of these instruments may be calibrated to read in terms of tem- 
perature, °C. 
Hlectric-resistance Pyrometer. A resistance pyrometer may be cali- 
brated by comparison with a calibrated instrument or by a number of 
known temperatures such as the boiling points of liquids or fusion points 
of metals, and in general three points are quite sufficient to completely 
calibrate the pyrometer, but no simple equation can be given for all metals. 
For platinum, however, the case is somewhat different, as an extensive 
study has been made of platinum resistance thermometry. 
Callender’ defined platinum temperature Pt as follows: 
R—Row 
tee) 
Rios—Ro 
where R = the observed resistance at the temperature t. 
where R = the observed resistance at 0° C. 
where R — the observed resistance at 100° C. 
® Proc. Roy. Soc. 41, p. 231, 1886. 
