1 86 COBLENTZ: NOTE ON COEI^'FICIENT OI^ RADIATION 



matter of fact, the crucial part of the apparatus was a receiver 

 with potential terminals attached thereto, at a sufficient distance 

 from the ends to avoid the question of heat conduction to the 

 electrodes. These potential wires, which were from 0.003 mm. 

 to 0.02 mm. in diameter, accurately defined the length of the 

 central part of the receiver which was utilized in the measure- 

 ments. By exposing the whole length of the receiver to radia- 

 tion, conduction losses did not enter the problem. The writer is 

 not aware of anyone having used a similar apparatus which 

 compares with this receiver in nicety of construction, and re- 

 producibility of results under given conditions. 



The receiver used by Kahanowicz was placed at the center 

 of a spherical mirror, with an opening in one side to admit radia- 

 tion. In this manner the correction for reflection was elimi- 

 nated. The shutter was close to the receiver. If its tem- 

 perature was different from that of the water-cooled diaphragm, 

 which was before the radiator, errors in the radiation measure- 

 ments would occur. As mentioned in my previous papers, 

 the shutter should be placed between the water-cooled dia- 

 phragm and the radiator, to avoid a change in surroundings 

 facing the receiver when the shutter is raised for making the 

 radiation measurements. The temperature range was from 

 260° C. to 530° C. The distance from the radiator to the re- 

 ceiver was 35 to 55 cm. A series of 28 measurements gave an 

 average value of o- = 5.61 X io~^- watt cm~'- deg~'. Of this 

 number 1 1 gave a value of 0- = 5.7. Out of a series of 4 measure- 

 ments made in Decemljer, 1916, with the distance d = ^6 cm., 

 three gave a value of cr — 5.7. 



No corrections were made for atmospheric absorption, which 

 for the temperatures used is not neghgible. In a previous 

 paper'' it was shown that on removing the moisture (va])or 

 pressure of 10 to 12 mm.) from a column of air 52 cm. in length, 

 the radiation constant was increased from a = 5.41 to 5.55 or 

 about 2.6 per cent. For the spectral region transmitted by 

 rock salt, to X = 15M, the absorption is about i percent.*^ Other 



* Bull. Bur vStand. 12: 576. 1916. See table 3, series CI,XXX to CLXXXII. 



* Proc. Nat. Acad. Sci. Loc. cit. 



