  I show that the average transverse momentum, $\langle p_t \rangle$, of the
hadrons emitted in relativistic nuclear collisions can be used as a ``knob'' to
control the strength of the magnetic field induced by the spectator and the
participant protons over the overlap region. I thus argue that any observable
sensitive to this magnetic field is nontrivially correlated with $\langle p_t
\rangle$ at a given collision centrality.
