ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 131 



platinum (0.3 and 0.6 per cent per degree Centigrade, respectively) have 

 been used. The semiconductor thermistors have appreciably larger, 

 negative coefficients of resistance (to —5 per cent per degree Centigrade) 

 (Becker et al., 1940) and are widely used (Dodd, 1951). Both thermistor 

 and metallic bolometers hav-e been made with a limiting sensitivity of 

 about 10~* watt, with a response time of a few milliseconds (Baker and 

 Robb, 1943; Jones, 1946; Billings, Barr, and Hyde, 1947; Billings, Hyde, 

 and Barr, 1947; Schlesman and Brockman, 1945), or increasing sensi- 

 tivity to 10"^" watt with response time of a few seconds (Jones, 1949). 



Thermocouple. By placing the absorbing surface in good thermal con- 

 tact with a bimetallic junction, a change in the temperature of the 

 absorber may be measured as a change in the potential difference across 

 the junction. To minimize the effects of ambient temperature changes 

 on such a thermocouple detector, this potential is usually measured with 

 reference to the potential across a similar junction, in thermal contact 

 with a second absorbing surface, adjacent to the first, but not exposed to 

 the radiation beam. If several such pairs of junctions are connected in 

 series to produce a larger total change in potential difference, the device is 

 called a thermopile. 



The potential difference thus developed may be amplified in a direct- 

 current amplifier, or it may be mechanically interrupted in a "breaker" 

 amplifier and thereby converted into an oscillatory signal to be amplified 

 in an alternating-current amplifier (Liston et al, 1946). Alternatively, 

 if the absorber has a small heat capacity, the radiation beam may be 

 mechanically chopped to provide a cyclic voltage. This latter method 

 has the additional advantage that it minimizes the effects of slow "drifts" 

 between the potentials of the measuring and reference junctions. 



The rate of change of potential difference across such a junction with 

 change of temperature is the thermoelectric power of the junction. The 

 highest thermoelectric powers are obtained with junctions between bis- 

 muth-antimony and bismuth-tin alloys (Pfund, 1937a; Hornig and 

 O'Keefe, 1947); with such junctions, powers of 120 mv/°C may be 

 obtained. However, because of the fragility of bismuth alloy junctions, 

 other metallic couples such as constantan-chromel (77 mv/°C) are occa- 

 sionally used (Launer, 1940). 



With such bimetallic couples and with careful design, it is possible to 

 measure a rise in temperature of the absorber of the order of 10~*°C. 

 Such radiation detectors can provide a sensitivity of 50 mv/mw or greater 

 (Schwarz, 1949; Jones, 1949). 



For use in photochemical experiments a thermopile with a large receiv- 

 ing area may be desired (Crane and Blacet, 1950). 



Golay Detector. The Golay radiation detector is characterized by both 

 a high sensitivity and a relatively rapid response time (Zahl and Golay, 

 1946; Golay, 1947a, b). The radiation is absorbed in a blackened surface 



