314 



In order to c;ilibrato or standardize a pyrometer it is necessary to have 

 a huninous source whose black body temperature is accurately known. The 

 primary standard must be some form of a heated inclosure whose walls 

 can 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 acHusted to maintain any desired temperature up to 

 about 1,600° C. 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 



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Fig. 9. 



center of the tube. If there is a cold junction it should be placed 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 reflned work the ice point is not necessary. The 

 furnace is connected in series with a rheostat and 110 v. 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 glohe incandescent lamp, which would 

 give a uniformly illuminated area of about 1 sq. cm., might be used if its 

 black-bndy temperature at some particular current strength were accu- 

 laiely known." 



For pyrometers like the Holborn-Kurlbaum (H.-K.), however, which 

 can only be calibrated empirically, it is necessary to have a black body 



8 Physical Review, Vol. 31, No. 4. Oct., 1910. 



