Alan Hodgkin and Andrew Huxley recorded the membrane potential directly between the inside of a squid axon and the external solution. The crucial result was an action potential that passed through zero: the interior became positive, rather than merely losing its negative resting membrane potential. A theory in which excitation simply abolishes membrane selectivity and shorts out the resting membrane potential cannot account for that reversal. Their original report establishes the historical interpretation independently of the unavailable examination picture.[ref]
Action Potentials Recorded from Inside a Nerve Fibre by Alan Hodgkin and Andrew Huxley. Nature 144 (1939), pp. 710–711: intracellular recording and reversal of the potential difference
The later Hodgkin-Huxley model explains the rising phase through increased sodium permeability and the falling phase through sodium ion-channel inactivation and increased potassium permeability. Distinct Nernst equilibrium potentials make a selective conductance change capable of reversing the membrane potential. The recording established the phenomenon; it did not by itself reveal individual ion channel structures or all their gating kinetics.
Source illustration TODO: the source page's recording is missing in both the held PDF and its archive parent. No amplitudes, time intervals, or trace layout are reconstructed here; the surviving historical request is answered above.
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