Gravitational Lens Systems to probe Extragalactic Magnetic Fields
  The Faraday rotation measurements of multiply-imaged gravitational lens systems can be effectively used to probe the existence of large-scale ordered magnetic fields in lensing galaxies and galaxy clusters. The available sample of lens systems appears to suggest the presence of a coherent large-scale magnetic field in giant elliptical galaxies somewhat similar to the spiral galaxies. 
D. Narasimha and S. M. Chitre