Abstract

While intensity of electromagnetic radiation (radio, infrared, light, or X-ray) gives us primary information about the distribution of the baryonic matter in the Universe, polarisation is a parameter that enables us to investigate many additional details. Polarisation at radio frequencies gives us details of emission processes since the non-thermal synchrotron process dominates at low radio frequencies in emission regions. The studies can be inverted and by means of polarisation observations the details of Cosmic Magnetic Fields in the emission regions can be delineated. The Zeeman Effect is based on polarisation observations and gives a method of direct determination of magnetic field. In addition polarised radio sources can be used as probes of the intervening medium through which the radio waves are propagated. Faraday Rotation effects are observed and in conjunction with known thermal emission can be used to determine magnetic fields. Some of the earliest radio polarisation observations were made in Cambridge in 1960-1963. For me the year 1963 was crucial: my PhD examination. From these early beginnings a whole research area has developed: the study of Cosmic Magnetic Fields. I will describe the technology needed for polarisation observations and show the early observations but in addition I will follow the developments up to the present day.