GENERATION, CONTROL, AND MEASUREMENT 255 



principle of electrical resonance, it is essential that the supply frequency 

 be very constant. For one commonly used type a 1 per cent change in 

 frequency causes a 1.8 per cent change in output voltage in the same 

 direction as the freciuency change. In the large interconnected power 

 systems of continental America, the average frequency is very constant, 

 as evidenced by the precision of electrical clocks. However, there are 

 short-time variations that may be as large as ±0.25 cps or 0.4 per cent. 

 This limits the short-time regulation accuracy to ±0.7 per cent, although 

 the long-time regulation will be very much better. Standard regulators 

 are usually designed to reduce a ± 17 per cent supply fluctuation to ± 1 

 per cent for a constant load. By operating two regulators in cascade so 

 that the first regulates the power supplied to the second, the average 

 long-time variation in voltage can be reduced to less than ±0.1 per cent, 

 but there is little improvement in short-time stability. When regulators 

 with electronic devices employing rectifiers are used, it is important that 

 the low-harmonic-distortion type be employed. The regulator supplies 

 constant root-mean-square voltage, whereas the voltage output from a 

 rectifier supplying a capacitor input filter is dependent on the peak value 

 of the voltage wave. The low-harmonic-distortion types are larger and 

 more costly, but they produce a pure sine wave relatively free of third- 

 harmonic distortion. 



The electronic a-c regulators emplo}^ a saturable reactor type of trans- 

 former controlled by a voltage-sensing element on the output and an 

 amplifier. One type has a diode whose filament temperature is dependent 

 on the output voltage. A change in filament emission causes a voltage 

 change which is amplified and controls the reactance of the power trans- 

 former in such a way as to oppose the change. These regulators have 

 the important advantage that they are not power-frequency-dependent 

 and are available with regulation control of 0.1-0.001 per cent. The har- 

 monic distortion is low, but the speed of regulation is not quite so rapid 

 as that of the magnetic regulators. 



REFERENCES 



Andrews, D. H., R. M. Milton, and W. DeSorbo (1946) A fast superconducting 

 bolometer. J. Opt. Soc. Anier., 36: 518-524. 



Barrows, W. E. (1951) Light photometry and illuminating engineering. McGraw- 

 Hill Book Company, Inc., New York. 



Becker, J. A., and H. R. Moore (1946) Thermistor bolometer detecting system for 

 infrared spectrometers. J. Opt. Soc. Amer., 36: 354-355. 



Bell, E. E., R. F. Buhl, A. H. Nielsen, and H. H. Nielsen (1946) Comparative 

 studies of the performance of infrared receivers. J. Opt. Soc. Amer., 36: 355. 



Billings, B. H., E. E. Barr, and W. L. Hyde (1947) Construction and character- 

 istics of evaporated nickel bolometers. Rev. Sci. Instr., 18: 429-435. 



Billings, B. H., W. L. Hyde, and E. E. Barr (1947) An investigation of the properties 

 of evaporated metal bolometers. J. Opt. Soc. Amer., 37: 123-132. 



