This paper describes an instrument for comparing the magnetic states of weakly magnetic materials, such as phosphor bronze, copper, filled rubber, etc. The force generated between a suspended magnet and the sample under test causes deflexion of a light beam. The instrument, as used at present, is suitable for comparisons in the range μ = 1 ± 0.00001 to μ = 1 ± 0.0002, but it can be adjusted to cover other values of μ. It has been found particularly useful for detecting magnetic contamination in fine gauge copper wire for instrument work.