WET DRIFT CHAMBERS FOR PRECISE LUMINOSITY

A set of high-precision compact drift chambers has been a vital compon ent of the OPAL luminosity monitor since the start of data-taking at L EP. They were augmented in 1992 by the addition of Small Angle Referen ce Chambers with a very similar design to the original chamber. The pe rformance of the chambers is reviewed, highlighting both the importanc e of using polyalkylene glycol (Breox) to maintain a uniform and paral lel electric field and the construction techniques used to sustain the required field strength. We describe some of the operating problems, with their solutions, and show how the chambers have been used in achi eving a systematic error of 0.41% on the luminosity measurement.