GENERATION, CONTROL, AND MEASUREMENT 225 



unbalance a-c emf being detected with an a-c amplifier, or (3) a two- or 

 four-arm bridge supplied with direct current but coupled to an a-c ampli- 

 fier that amplifies the a-c component of the signal produced by modulating 

 or chopping the incident flux. 



The d-c bolometer bridge and galvanometer operating at high sensi- 

 tivity always exhibit an uncertain drift of the zero position. This is 

 principally due to fluctuations in thermoelectric emf and resistance caused 

 by minute temperature variations occurring in various parts of the cir- 

 cuit, and to variations in stray radiant flux. 



The zero drift due to slow changes in the thermal emf at each junction 

 of the bolometer circuit can be eliminated by supplying the bridge with 

 alternating current and detecting the unbalance with a conventional a-c 

 amplifier that is tuned to the frequency of the bridge power supply (Moon, 

 1935; Schlesman and Brockman, 1945). The sensitivities obtainable are 

 equivalent to those of the most sensitive galvanometer systems and are 

 limited only by the thermal, or Johnson, noise in the resistance elements 

 of the bridge. However, this system does not eliminate zero drift due to 

 small fluctuations in resistance of the bridge components or fluctuations 

 in stray flux. 



Zero instability due to fluctuations in resistance, thermoelectric emf, 

 and stray flux can all be reduced to second-order variations by modu- 

 lating or chopping the radiant flux at the source and using a fast bolometer 

 in an a-c bridge circuit connected to a tuned amplifier. Modulation fre- 

 quencies of 5-30 cps are commonly used. It is important to recognize 

 that fluctuations in stray flux are eliminated from the signal only if the 

 source itself is modulated and the stray flux is not. The chopping device, 

 which frequently consists of a rotating sector, should be placed at the 

 source and not at the detector. 



The metal bolometer, with its low impedance and voltage sensitivity, 

 requires a high-turns-ratio coupling transformer for impedance matching 

 it to the grid circuit of an a-c vacuum-tube amplifier (Fig. 3-25). Turns 

 ratios of 100-750 have been used with metal bolometers and thermo- 

 couples (Robinson, 1952). Thermistor bolometers are almost universally 

 used with chopped or modulated incident flux at frequencies up to 15 cps. 

 The thermistor-bolometer bridge (Fig. 3-25) consists of only two arms 

 instead of the usual four, since only the a-c component is amplified. The 

 bridge voltage is usually between 100 and 400 v do. The minimum 

 detectable power, as limited by signal/noise ratios, is of the same order 

 of magnitude for both metal and thermistor bolometers when properly 

 coupled to vacuum-tube amplifiers. 



Thermocouple and Thermopile. The radiation thermocouple consists of 

 a pair of thermoj unctions in which one, the hot junction, is in contact 

 with a blackened receiver. The cold junction is connected either to a 

 similar receiver, which may be used as a compensating element for stray 



