The influence of the magnetic loop stretching with a longitudinal electric current on the change of the electric current and its magnetic energy is studied. The electrodynamical forces in the magnetic loop are always directed so that they increase its inductance, namely, they stretch the magnetic loop. The velocity of the magnetic loop stretching depends on the electric current value. The work of these forces spends for increasing the plasma kinetic energy. Increasing the magnetic loop length decreases of the current and its magnetic energy. The magnetic energy of electric currents includes the mechanical work of electrodynamical forces, Joule heating and the work of exterior electromotive forces. The most part of the consumable magnetic energy is expended for the mechanical work of the electrodynamical forces.