A grounded metal shield that is around a signal wire prevents outside electric fields from reaching the signal wire. This is because electrons are free to move in a metal and the electrons in the metals surface move in response to the outside electric field. Below the metal surface the electric field is canceled by the field created by the electrons that have been drawn to the metal surface by the outside field, which is the noise source. Since the signal wire is within the metal cage it does not receive the noise.