Measurements of the wavelength dependence of the linear polarization of twenty-nine radio sources have been compiled. It is confirmed that in the decimeter wavelength range the observed Faraday rotation of the plane of polarization arises in the Galaxy. Although the magnetic field in the local spiral arm must be highiy ordered, the data do not allow a definite field configuration to be deduced. The simple model due to Chandrasekhar and Fermi (1953) is shown to be untenable, however. The hypothesis that depolarization at decimeter wavelengths is caused by differential Faraday rotation in the Galaxy is rejected for lack of supporting evidence. An inverse correlation between emissivity (or magnetic-field strength) and fractional polarization at 10-cm wavelength is presented. This is interpreted as confirmation of the suggestion by Woltjer (1962) that depolarization is produced by Faraday rotation effects within the radio sources themselves. Sources with high magnetic fields will therefore only exhibit detectable polarization at centimeter wavelengths. At these wavelengths they will have large depolarization rates and consequently large rotation measures due to their internal Faraday rotation. Such sources,in contrast to the majority of those studied hitherto, will therefore show poor correlation between rotation measure and galactic latitude and an enhanced correlation with depolarization rate.