  We compared magnetograms from the KPVT/SPM, SoHO/MDI, SOLIS/VSM, and SDO/HMI
with the aim of probing the effect on measured solar magnetism of the variation
in instrument response with time, magnetogram signal level, and position on the
solar disc. Taking near-simultaneous observations from the various instruments,
we examined the surface coverage by magnetic activity and the effect of
cross-calibrating the various instruments under different assumptions. By
comparing the surface coverage by magnetic activity in the observations from
the various instruments, we traced the effect of the time variation in
instrument response on the longitudinal magnetogram signal and disc-integrated
unsigned magnetic flux. This yielded evidence of acute changes in the response
of MDI and VSM with certain events such as the SoHO vacation in 1998 and the
upgrade of the VSM CCD camera in 2009. Excluding these changes, the effect of
instrument instability on the magnetogram signal and disc-integrated magnetic
flux appears to be rather benign, with an associated uncertainty of less than
2%. We determined the magnetogram signal ratio between each instrument pairing
as a function of magnetogram signal level and distance from disc centre and
with it cross-calibrated the various instruments. We compared the result with
that from repeating the cross-calibration with the overall magnetogram signal
ratio. This allowed us to estimate the uncertainty in the magnetogram signal
associated with the variation in instrument response with magnetogram signal
level and distance from disc centre to be about 8% to 14%. The corresponding
uncertainty in the disc-integrated magnetic flux is about 7% to 23 %. The
results here will be useful to the interpretation of SPM, MDI, VSM, and HMI
magnetograms. As examples, we applied our findings to selected results from
earlier studies based on such data.
