GENERATION, CONTROL, AND MEASUREMENT 



227 



powers known, but the specific resistance of the semiconductors is so high 

 that a pin type of mounting must be used. The ultimate sensitivity 

 obtainable is not much greater than that for the all-metal couples (Liston, 

 1947). 



Fast thermocouples are obtainable by using extremely fine wires welded 

 or soldered to thin receivers of metal foil and by evaporating alloys onto 

 thin films of supporting materials such as nitrocellulose, as in the con- 

 struction of evaporated bolometers. Both types of construction have 

 yielded low time constants. Comparison of several typical thermoelectric 

 detectors is given in Table 3-16. 



RADIANT FLUX 



I I I I I 



M I M 

 + + + >(♦ 



CONSTANTAN 



BLACKENED 

 RECEIVER 



LARGE 



THERMAL 



MASS 



RADIANT FLUX 

 I I 



I I I I I 



+ V + y y I + 



CONSTANTAN 

 WIRES 



ih) 



CONTINUOUS FLUX 



Mill' 



' ' ' ' ' ^RECEIVER 



t t t t U^ 



- + 



T> 



llkJ 



cw^w\ 



SEMICONDUCTOR 

 ALLOY TIP 



NOBLE METAL PIN 



CONTACT 

 MODULATOR 



COUPLING 

 TRANSFORMER 



TO AC AMPLIFIER 



MODULATED FLUX 

 I I I I I 

 I I I I I 



y t i > t ^NITROCELLULOSE 



BACKING FILM 



^\ 



EVAPORATED 

 METAL FILMS 





TO AC AMPLIFIER 



(c) id) 



Fig. 3-26. Diagram of various types of thermoelectric detectors and coupling cir- 

 cuits. An imcompensated constantan-copper wire thermocouple is shown at a. 

 The linear thermopile of h is partially compensated by attaching the cold junctions 

 to blackened receivers. The radiant flux being measured is focused on only one set of 

 receivers. A compensated pin-type thermocouple is shown at c. Two couples and 

 receivers are connected in electrical apposition. The output is converted to alter- 

 nating current by the contact modulator and coupled to a tuned amplifier through a 

 high-turns-ratio transformer. The evaporated film-type thermocouple of d has a 

 short time constant and responds to flux modulated at low frequencies. 



