Polarization of the microwave background: experiments.

We review the available results on Cosmic Background Polarization (CBP): we discuss the advantages and disadvantages of ground based observation of CBP versus satellite observations. Atmospheric noise can be reduced to a very small amount by an appropriate design of a polarimeter. Various polarimeters are discussed and the spurious polarization of optical components is described. Finally we present a new type of polarimeter with spurious polarization smaller than 10-7 and capable of measuring CBP at angular scales of 5-10 arcminutes from ground