VISIBLE AND NEAR-VISIBLE RADIATION 143 



being produced, as indicated by thermocouples. In general, this type 

 of instrument has not been constructed of sufficient sensitivity to be 

 used for sources furnishing markedly less radiant energy than the sun. 



COMPARISON WITH BLACK-BODY EMISSION 



A second method of absolute determination in\'olves the use of a 

 black-body receiver whose temperature can be measured by a suitable 

 device, wherein the deflection is simply proportional to the energy falling 

 upon the receiver. With such a secondary type of instrument, yielding 

 only a deflection proportional to total energy, it is necessary that its 

 deflection be calibrated by some other method, either by means of a 

 known source, such as a black body at known temperature, or by com- 

 parison with an absolute instrument. In practice, a standard incan- 

 descent lamp, generally carbon filament, is commonly used as a known 

 source. Such a lamp, properly seasoned, may be secured from the Bureau 

 of Standards, or a suitable lamp may be calibrated by them (23). It is 

 only necessary for this purpose that the total radiation in a given direction 

 be known for a specified condition of operation. One of the simplest 

 instruments of the secondary type is the silver-disk pyroheliometer (2). 

 It consists of a high-grade mercury-in-glass thermometer, bedded in a 

 silver disk whose surface is blackened and the area accurately determined. 



Another widely used instrument is the bolometer (3). Here two 

 identical blackened strips of metal are placed in an evacuated space. 

 The two strips become the two arms of a Wheatstone's bridge, whose 

 balance is determined by a galvanometer. When radiation is allowed 

 to fall on one of these strips, a rise in temperature is produced which 

 unbalances the bridge by change of resistance. The chief difficulty with 

 this type of instrument is that it requires a constant current to be passed 

 through the two strips, thus necessitating a constant source of potential. 



A third form, by far the most commonly used, is simply a thermo- 

 couple with a blackened receiver. Where two dissimilar metals join, an 

 electromotive force is developed which depends upon the temperature. 

 Thus, if a circuit is made up of two parts, one copper and the other 

 constantan, electromotive forces will be developed in opposite directions 

 at the two junctions. If these are at identical temperatures, no current 

 will flow. If, however, a metallic receiver is attached to one of the 

 junctions and its surface blackened, its temperature may be raised by 

 radiation. For small temperature changes, the unbalanced electromotive 

 force is proportional to the temperature rise and hence to the radiant 

 power impinging upon the receiver. If a galvanometer is so constructed 

 as to produce a deflection proportional to the current, the combination 

 of thermocouple and galvanometer yields a deflection strictly propor- 

 tional to irradiation. 



