  With the increasingly common use of large area PMTs (photomultiplier tubes)
for nuclear and particle physics experiments, information on the position
dependent magnetic field effects across the PMT's photocathode is important to
effectively analyze and understand data collected from PMTs. Using an automated
two-dimensional scanner, we have measured the PMT response to an external
magnetic field as a function of the cathode position impinged by a collimated
light source. This study shows a clear dependence on the cathode position in
both collection efficiency and gain of the PMT in the presence of a magnetic
field. In particular, a pronounced valley in the collection efficiency is
observed for certain locations of the cathode when the magnetic field is
transverse to the PMT axis. The pattern of the position dependence is presented
for several different magnitudes and orientations of the magnetic field
relative to the PMT.
