The most valuable new information on galactic magnetic fields is derived from the study of Faraday effects in extragalactic radio sources and pulsars. The rotation of the plane of polarization of an electromagnetic wave with a wavelength of 1 m – the so called Rotation Measure RM – which traverses a medium with electron density n e (cm−3) and magnetic field intensity H (in μG) is given by where dl is the element of length along the line of sight expressed in pc. In the case of the pulsars, the delayed arrival of the pulses at the longer wavelengths immediately yields the integrated electron density along the line of sight, that is