It has been observed that the microwave power loss of a section of wave guide filled with iron powder material could be varied by subjecting the iron powder to an externally applied magnetic field. This paper describes some experiments performed with various kinds of iron powders. Measurements have been made with the magnetic field applied parallel to, and perpendicular to the plane of the magnetic component of the electromagnetic wave inside the guide. The possibility of ferromagnetic resonance is considered. A theoretical formula for the power loss exhibited by low conductivity iron powder attenuators is developed and compared with experimental measurements.