A long base-line interferometer has been operated at 18 cm over an 845-km (4.7 X 10~ X) base line between the NRAO 140-foot radio telescope at Green Bank, West Virginia, and the 120-foot telescope at the Haystack Microwave Facility near Boston, Massachusetts. Independent rubidium-controlled atomic clocks were used as time and frequency standards at each telescope. The intermediate-frequency signals from each antenna were recorded on magnetic tape and later correlated in a digital computer Reliable data were obtained for twenty-eight sources. Eleven appear unresolved and are therefore less than O'~O1-O'~O2 in diameter. Four others have components that contain a significant fraction of the total flux in a component less than this size; and two others are completely resolved and thus do not contain significant small-scale structure. The ten partially resolved sources have angular dimensions of the order of a few hundredths of a second of arc. Comparison of the radio-frequency spectra with interferometer measurements made at several wavelengths indicates that many sources consist of a number of well-defined, small-diameter compo- nents, each of which has a diameter independent of wavelength. The magnetic field deduced from the low-frequency cutoff in the radio spectra is of the order of 1O-~ gauss in the partially resolved compo- nents
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Clark et al. (1968) studied this question.