142 BIOLOGICAL EFFECTS OF RADIATION 



For convenience in calculating the brightness of a black body of 

 specified temperature 6, an empirical formula has been evolved (67) 

 which is in excellent agreement with the calculated and observed values 

 over the range from 1800° to 2728° K. 



461.4 



B = 24951 



•<24) 



Table 8 gives the brightness of the black body for various 

 temperatures. 



MEASUREMENT OF TOTAL RADIATION 



ABSOLUTE DETECTOR 



Instruments which yield an absolute measurement of radiation apply 

 a calorimetric method to the black body. The pyroheliometers used 

 for the standardization of solar-radiation measurements represent this 

 type of instrument. The water-flow pyroheliometer used by Abbot (7) 

 will serve as an illustration. It consists of two identical blackened 

 chambers through which water can be circulated, one stream entering 

 each chamber at the same temperature. Radiation is allowed to fall 

 upon one chamber, producing a rise in temperature in the water stream. 

 An equivalent rise in temperature is produced by a known electrical 

 current through a known resistance in the other chamber. An electrical 

 thermometer is used to secure equality in the temperature of the two 

 identical streams. The heat supplied electrically to the second chamber 

 is thus equal to the heat supplied by radiation to the first and can be 

 evaluated either in calories per second or in watts. Knowing the area 

 of the receiver, the irradiation can thus be determined. In practice, the 

 influence of the surroundings is minimized by surrounding the chambers 

 with an evacuated space, thereby eliminating convection. Since radia- 

 tion loss from the two chambers is identical, it does not enter into the 

 calculation. 



A second type of absolute detector is the melikeron pyranometer (8). 

 In this form, a metal strip blackened and so shaped as to constitute 

 effectively a hollow inclosure is heated by radiation. The rise in tem- 

 perature is measured by a thermocouple-galvanometer system. An 

 equivalent rise in temperature is then produced electrically and the heat 

 supplied determined, so that in this respect it is equivalent to the pyro- 

 heliometer, in that energy rate of supply is measured under presumably 

 identical conditions. 



o 



A compensation pyroheliometer (9) has been constructed by Ang- 

 strom, with two identical blackened strips, one of which is heated by 

 radiation and the other by electrical energy, identical rises in temperature 



