The discovery of powerful X-ray sources associated with rich clusters of galaxies has led to the prediction that these clusters contain large amounts of hot gas. A necessary consequence of the existence of such gas is the Compton scattering of cosmic microwave background radiation by hot electrons (Sunyaev & Zeldovich 1972). As a result, some of the microwave photons are re-distributed with slightly higher energies and the effect may be observed as a small diminution in the background temperature. The effect is expected to be ≈0.5 mK for the gas clouds associated with cluster X-ray sources.

Previous attempts to detect this effect have not been successful. I present here the results of two further attempts; by Lake & Partridge using the 36’ telescope at Kitt Peak, and by Gull ’ Northover (1976) using the 25-m telescope at the SRC Chilbolton Observatory. Both groups used similar, twin-beam systems, comparing the temperature of the background radiation in the directions of various known X-ray clusters with that at points ≈15 arcmin distant. In order to minimize the effect of non-thermal background sources, it is desirable to observe at the highest frequency compatible with atmospheric stability. Gull & Northover, using a frequency of 10.6 GHz, accumulated 670 hours of observations, whilst Lake & Partridge, who did not have to contend with British weather, selected the more difficult frequency of 33 GHz, but could only manage 80 hours integration.