As supernova remnants expand their shock waves are freezing in and compressing the magnetic field lines they encounter; consequently we can use supernova remnants as magnifying glasses for their ambient magnetic fields. I will describe a simple model to determine emission, polarization, and rotation measure characteristics of adiabatically expanding supernova remnants and how we can exploit this model to gain information about the large scale magnetic field in our Galaxy. I will give two examples: The SNR DA530, which is located high above the Galactic plane, reveals information about the magnetic field in the halo of our Galaxy. The SNR G182.4+4.3 is located close to the anti-centre of our Galaxy and reveals the most probable direction where the large-scale magnetic field is perpendicular to the line of sight. This may help to decide whether the large-scale field in our Galaxy is axissymmetric.