  Several mathematical formulae are used nowadays in order to compute a
magnetic torque. We demonstrate that its more general expression is the
vectorial product of the current density with the vector potential. We
associate this Larmor's torque with Amp\`{e}re's force and more specifically
with Helmholtz mechanical tension, which is at the origin of the longitudinal
stresses in "open" circuits carrying current. We show that Amp\`{e}re's force
enters into the realm of Newtonian Electrodynamics and we explain the absence
of contradiction with special relativity. Hence, we provide for the first time
a theoretical basis for the numerous experiments, which claimed to have
demonstrated the existence of the longitudinal mechanical tension starting with
the historical Amp\`{e}re's hairpin demonstration and the more modern ones of
the Graneaus and of Saumont.
