146 BIOLOGICAL EFFECTS OF RADIATION 



placed in a gas-filled space at somewhat reduced pressure. When the 

 surface of one receiver is heated by radiation, convection currents are 

 set up which produce a deflection. It is important in this type of instru- 

 ment that the front and back surfaces of the receiver be at as great a 

 temperature difference as possible. For this reason, the receivers are 

 generally laminated in character. Thus, mica is sometimes employed. 

 The mica is subjected to a pearling process, producing a multiplicity of 

 cleavages between the separate layers of the crystal. 



A very sensitive radiometer has been constructed by Abbot wherein 

 the receiver was built up of several layers of fly's wings (5). With such 

 an instrument, it has been possible for him to measure the spectral 

 distribution of radiation from stars, using the 100-in. telescope at Mt. 

 Wilson. 



In order to secure a maximum sensitivity in radiometer measurements 

 it is necessary that optimum gas pressure be maintained. For details 

 regarding the construction of such instruments, the reader is referred 

 to a more complete discussion by Coblentz (19). A recent discussion of 

 the theory has been given by Marsh, Condon, and Loeb (48). 



COMPARISON OF DETECTORS 



As a matter of convenience in comparing the sensitivity of the different 

 types of instruments available for radiation measurements, and as a 

 guide in the selection of suitable current-measuring devices. Table 9 has 

 been prepared. For spectroscopic purposes, a small narrow receiver is 

 desirable. If continuously recording methods are required, a small time 

 of response is at a premium. Thus, in comparing the different instru- 

 ments available, it is important to take these characteristics into account. 

 Thermocouples referred to in Table 9 have been selected because of their 

 small areas of receiver. If radiation is to be measured over an extended 

 area, thermocouples can be constructed having much larger areas of 

 receiver, the sensitivity to irradiation increasing roughly in proportion to 

 the area. For a small-area instrument, the single couple is superior to any 

 multiple-junction thermopile so far constructed. 



Table 9 is divided into three parts, the first dealing with the character- 

 istics of the detector unit, the second with the characteristics of the 

 measuring device, galvanometer, amplifier, etc., and the third with the 

 sensitivity, (a) of the active element, and (6) of the overall combination. 

 For instance, the sensitive thermocouple has a resistance of 17 ohms, an 

 area of 0.003 cm. 2, an inherent time of practical equilibrium of 0.4 sec. 

 A suitable galvanometer to be used with this couple should have prac- 

 tically the same resistance. If it has a 5-sec. time of response, it will have 

 a voltage sensitivity of 2 X 10~^ and a current sensitivity of 4 X 10~^^ 

 amp. for the least measurable deflection under favorable conditions. 



