GENERATION, CONTROL, AND MEASUREMENT 



223 



and compensator facing in the same direction so that either element can 

 be used as the receiver and both elements receive the same scattered flux. 

 The slit image is then focused on only one element. When measuring 

 the flux from a large source, it is not possible to produce a small image, 

 and the compensator must then be shielded internally or placed behind 



-RECEIVER 



-COMPENSATOR 



RECEIVER 



TUNED 

 ELECTRONIC 

 AMPLIFIER 



HIGH TURNS 

 RATIO COUPLING 

 TRANSFORMER 



D-C BRIDGE 

 ~ SUPPLY 



THERMISTOR 



COMPENSATOR 



FLAKE 



THERMISTOR 

 RECEIVER 

 tr^a FLAKE 



FLUX 

 MODULATOR, 

 15 CYCLES 

 PER SECOND 



-5 + 



HIGH GAIN AMPLIFIER 

 TUNED TO 15 CYCLES 

 PER SECOND 



(c) 



Fig. 3-25. Diagram of the radiation bolometer and coupling circuits for continuous and 

 modulated flux. Arrangement a is the classical system employing a galvanometer. 

 All four arms of the bridge have approximately the same resistance. R^ varies the 

 sensitivity of the compensator arm so that both receiver and compensator have equal 

 sensitivities to stray flux. The arrangement at b is suitable for chopped or modulated 

 flux. The low-resistance metal bolometer is coupled to fhe tuned amplifier through a 

 high-turns-ratio transformer. The high-resistance thermistor bolometer may be 

 coupled directly to the grid circuit of an amplifier through a capacitor, as shown at c. 



the receiver. These arrangements efi"ectively compensate for fluctu- 

 ations in ambient temperature but do not compensate for fluctuations 

 in scattered flux. 



1. Metal. The receivers of metal bolometers consist of thin strips of 

 metal produced by rolling fine wires, by electrodeposition on a backing 

 plate such as thin sheet copper which can be dissolved away by appropri- 

 ate reagents (Brockman, 1946), and by vacuum evaporation or sputtering 

 of metal on thin backing films of a plastic (Billings, Barr, and Hyde, 

 1947). Ribbons produced by roUing wire or by electrodeposition can be 

 produced and handled as thin as 0.1 n. By evaporating the metal in a 



