The monitor spectrometer is used to monitor the stability of the high voltage of the main spectrometer. Like the main spectrometer, it is operated as a MAC-E-Filter. It measures the energy of conversion electrons from the Krypton-83m decay, which have always the same energy. Basically it is an almost 10m long, very accurate voltmeter, using a nuclear standard as a reference.
Huge precision "voltmeter" using the former Mainz spectrometer setup.