A system for compensating for magnetic disturbances caused by the rotating stirrer magnet of a photometer sample cuvette on the photomultiplier tube of the photometer. To obtain the required compensation, a companion similar magnet is located coplanar with and adjacent to the main stirrer magnet and is counter-rotated synchronously with the main magnet in opposite magnetic phase therewith. The two counter-rotating magnets are physically located symmetrically with respect to the photomultiplier tube so that their magnetic effects on the electron flow of the photomultiplier tube cancel each other out. The companion magnet may be the stirrer magnet associated with a reference cuvette mounted in side-by-side relation to the sample cuvette. The counter-rotating magnets may be gearingly coupled together and may be mounted coaxially with respective ejection plungers associated with the cuvettes.
