Figure 1 shows the main components of the experiment: a system of two pendulums that form a microwave resonator, similar to a Fabry-Perot interferometer. The introduction of external field masses causes a tiny acceleration of the pendulums in the direction of the resonator axis. The resulting change of the resonator length caused a measurable change in the resonance frequency.

The resonator was located inside a vacuum vessel. A transport system moved the eld masses between positions A and B.

An earlier version of the apparatus had been built and operated at Wuppertal University for a precision measurement of the gravitational constant: [8], [9], [10], [11], where details can be found. It was later transferred to DESY to an experimental hall of the PETRA accelerator.

After an initial measurement period, which resulted in a publication [6], it was relocated to an underground experimental hall of the HERA accelerator, and the setup was improved signi cantly.