314 
In order to calibrate or standardize a pyrometer it is necessary to have 
a luminous source whose black body temperature is accurately known. The 
primary standard must be some form of a heated inclosure whose walls 
ean be maintained at a uniform constant temperature. Some means must 
also be used for determining the true temperature of the inside of the 
inclosure. 
This is generally accomplished by some form, of an electric re- 
sistance furnace, as shown in Fig. 9. It consists of a central porcelain 
tube overwound with thin platinum foil through which passes an electric 
current which can be adjusted to maintain any desired temperature up to 
about 1,600° €. Concentric with this tube are two shorter ones which, 
with the intervening air spaces, minimize the radiation. Some form of 
thermo-couple is placed in one end so that the hot junction is near the 
Fire. 9. 
center of the tube. If there is a cold junction it should be piaced in 
crushed ice. The thermo-couple may be either connected to a potentiome- 
ter or a sensitive potential galvanometer which reads millivolts, and 
by means of a previously determined calibration any temperature may be 
determined. Except in refined work the ice point is not necessary. The 
furnace is connected in series with a rheostat and 110 y. DC. 
For the calibration of the Wanner or Le Chatelier pyrometer it is 
not necessary, as will be shown on the following pages, to know but one 
black-body temperature so that as a working standard any convenient 
luminous object such as a frosted globe incandescent lamp, whieh would 
give a uniformly illuminated area of about 1 sq. cm., might be used if its 
black-body temperature at some particular current strength were accu- 
rately known.* 
For pyrometers like the Tlolborn-Kurlbaum (IL-K.), however, which 
can only be calibrated empirically, it is necessary to have a black body 
8’ Physical Reyiew, Voi. 31, No, 4. Oct., 1910, 
