  Recent theoretical work has highlighted that quantizing a superconducting
circuit in the presence of time-dependent flux $\Phi(t)$ generally produces
Hamiltonian terms proportional to $d\Phi/dt$ unless a special allocation of the
flux across inductive terms is chosen. Here, we present an experiment probing
the effects of a fast flux ramp applied to a heavy-fluxonium circuit. The
experiment confirms that na\"ive omission of the $d\Phi/dt$ term leads to
theoretical predictions inconsistent with experimental data. Experimental data
are fully consistent with recent theory that includes the derivative term or
equivalently uses "irrotational variables" that uniquely allocate the flux to
properly eliminate the $d\Phi/dt$ term.
