SEARCH FOR HIGH ROTATION MEASURES IN EXTRAGALACTIC RADIO-SOURCES .1. MULTICHANNEL OBSERVATIONS AT 10 GHZ

Multi-channel polarimetry has been performed to detect high rotation m easure (RM) at 3 cm using the Nobeyama 45-m telescope. The high RM can didates of 96 radio sources were selected to be observed, and RMs of 3 5 sources were derived from the observations. Since the four channels are set contiguously from 2.84 cm to 3.31 cm, \RM\ can be derived uniq uely up to 15000 rad m(-2) by this polarimeter. We found that there ex ist sources with RM of several thousands rad m(-2). In fact, 5 sources have \RM\ > 1000 rad m(-2) On the other hand, all sources observed ar e well within this system limits, and therefore we suggest the observe d upper limit of \RM\ is around 5000 rad m(-2) for extragalactic radio sources, even taken into account the redshift of sources.