GENERATION, CONTROL, AND MEASUREMENT 



243 



instruments. Of the several systems that have been developed for con- 

 verting direct to alternating current, the two most commonly used are 

 contact modulation and capacity modulation. Contact modulation 

 involves either a magnetically driven vibrator or a motor-driven switch- 

 ing device for chopping the d-c signal of the detector and converting it 

 into an alternating current. The contact-modulation system is especi- 

 ally suitable for low-impedance detectors, such as thermocouples and 

 bolometers. A motor-driven system employing two sets of gold con- 

 tacts, one for converting direct to alternating current and the other for 



Table 3-18. Comparison of Characteristics of Sensitive Electrical 



Measuring I.xstri.ments 



synchronous rectification, has been described by Liston et al. (1946). 

 Commercial instruments have such a low inherent noise input from the 

 amplifier and contacts that the limiting measurable voltage is determined 

 by Johnson noise in the resistance of the detector. 



The capacitor system of modulation in the form of the vibrating-reed 

 electrometer has been described by Palevsky et al. (1947) and Reese (1952) 

 for the measurement of ionization currents. It is a. high-impedance 

 device and ideally suited to the measurement of the output of the vacuum 

 photoelectric cell and the photomultiplier. Commercial recording instru- 



