An experiment of impulsive electrodynamics [Eur. Phys. J. D 15, 87 (2001)] has been interpreted by the experimenters as a confirmation of Ampère’s law, because they consider the force exerted on a mobile section as due only to another small section. The integration over all the circuit gives zero longitudinal force by both Ampère’s and Grassmann’s laws. The correct interpretation of the experiment comes from analyzing the contributions of the different air pressures in the two air gaps due to different solid angles for radiation and particle losses during the electrical discharge. Moreover, there is a larger number of ions hitting the bases of the smaller gap because of a larger useful solid angle. Finally, ions are more trapped in the smaller gap because of a larger number of bounces. This interpretation leads to a better agreement with the experimental results.