Students can obtain further insight into the physical essence of the principle of relativity if they
experimentally investigate the phenomenon of electromagnetic induction in various reference frames.
For this purpose we propose a special apparatus. This device is an indicator of a potential
difference. The use of the apparatus makes it possible to detect an electric field in a reference
frame moving uniformly and rectilinearly relative to a permanent magnet in a uniform magnetic field,
which is created by this magnet. In addition to the above, the indicator of a potential difference
ensures the fulfilment of a number of demonstration experiments on electrodynamics.