A brief description of an Ampe`re electrodynamic experiment is given and some connection with it is indicated in such effects as those of: Wiedemann, Matteucci (MS), and Hall. Experimental arrangements and procedures in the MS study of ferromagnetics is described. The essential differences in the MS of different ferromagnetics are cited. An interpretation of the MS has been based on the well‐known effect of the exchange energy in ferromagnetic metals dependent on the interatomic distances instead of a phenomenological interpretation in terms of the magnetostriction.