234 BURGESS AND FOOTE : RADIATION PYROMETERS 



the sun. The auxiUaiy apparatus, galvanometer or potentiome- 

 ter, and recording devices, can be constructed so that their er- 

 rors will be practically negligible. For the work of the highest 

 accuracy the potentiometric method of measurement is to be 

 preferred with thermoelectric radiation pyrometers. 



The principal errors to which the several types of radiation 

 pyrometers are subject are shown to lie in the design and me- 

 chanical construction of these instruments, and certain of these 

 inherent errors, such as lag or slowness in reaching an equilib- 

 rium reading, require for satisfactory results, that the pyrometer 

 be calibrated and used under similar conditions of time of ex- 

 posure, distance from and aperture of source. Wide variations 

 in the lag effect exist among apparently similar instruments, 

 ranging from a few seconds to an hour or more. These and 

 other errors of appreciable magnitude, such as stray reflection, 

 convection currents, intervening atmosphere, size of source, 

 tarnishing of receiving mirror, etc., should be considered in 

 accurate work. It is shown that errors greater than 100°C. 

 may readily be caused by dirt on, or oxidation of, mirror. The 

 magnitude of errors due to varying the focusing distance, may 

 amount to several hundred degrees if suitable precautions are 

 not taken. 



A convenient method for the rapid comparison of different 

 types of pyrometer, and for determining the effects of size of 

 aperture and focusing distance was devised, consisting of a 

 wide nickel (oxide) strip heated electrically, a series of circular, 

 water-cooled diaphragms, and an optical bench. 



The methods in use at the Bureau of Standards for the calibra- 

 tion of radiation pyrometers by means of especially designed 

 experimental "black bodies" are described in detail and methods 

 of extrapolation outlined, as well as methods of use of radiation 

 pyrometers, including methods of obtaining approximately 

 correct temperatures for the case in which a source of insufficient 

 size is sighted upon. The application of the radiation pyrometer 

 to the determination of the total emissivity of non-black sub- 

 stances and to the measurement of temperatures is also discussed 

 in detail. 



