  Parallel circuit and the Lorentz forces on current carrying wires are
important concepts in introductory physics courses. Here we describe an
experiment that illustrates these two concepts. We mount a circuit with
multiple grounding points onto a torsion balance. We show that the grounding
points create parallel return paths for the supply current. When the topology
or the shapes of the return paths are altered, the Lorentz forces exerted by
the currents in the return paths within a magnetic field change accordingly,
which in turn cause changes in the rotary displacement of the torsion balance.
This experiment is simple and can be easily reproduced in a teaching
laboratory. What makes it interesting to students is that recently two research
teams have attempted to detect thrusts from microwave driven asymmetrical
resonance cavities (EmDrive or Cannae Drive), and the phenomenon observable in
this experiment provides an alternative explanation to the thrusts they
detected.
