Measurement of unambiguous rotation measures of extragalactic sources.

The results are reported of polarization measurements conducted specifically to check for the existence of very high Faraday rotation measures, which might otherwise have been undetected. Polarization measurements are presented for 24 extragalactic radio sources at 4 contiguous bands around 9 cm, from observations at the NRAO 300 ft. telescope. It is shown how these measurements can be used to derive rotation measures (and limits) which are free from n pi phase ambiguities around 9 cm. It is found that the data can not be made consistent with arbitrarily large rotation measures. Revised rotation measures and initial position angles are presented for the observed sources.