  In this paper we report a detailed investigation of a number of different
materials commonly used in precision instrumentation in the view of using them
as critical components in the magnetic gradiometer. The materials requirement
inside a magnetic gradiometer is stringent because the presence of magnetic
susceptible material will introduce intrinsic errors into the device. Many
commercial grade non-magnetic materials still have unacceptably high levels of
volume magnetic susceptibility between 0.001 and 0.0001. It is shown here that
machining with steel tools can further increase the susceptibility by up to an
order of magnitude. The ability of an acid wash to remove this contamination is
also reported. Washing in acid is shown to reduce the variation of volume
susceptibility in several commercial grade plastics which already have low
values of susceptibility.
