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Radio spectra are shown for thirty radio sources that show evidence of synchrotron self-absorption. As a result of inverse Compton cooling and adiabatic expansion, all opaque components have a maximum brightness temperature in the range 10h1₁012 ° K. The magnetic field deduced from the surface bright- ness and cutoff frequency in these sources is generally about 10-4±1 gauss. The variable components appear to have somewhat stronger magnetic fields
Kellermann et al. (1969) studied this question.