= Intracellular recording of membrane potential
In 1939, <Alan Hodgkin> and <Andrew Huxley> reported intracellular <action potential> recordings from squid giant <axons> at Plymouth.<Action Potentials Recorded from Inside a Nerve Fibre by Alan Hodgkin and Andrew Huxley>[Action Potentials Recorded from Inside a Nerve Fibre, Nature 144 (1939), pp. 710–711: inspected publisher abstract, preparation and intracellular recording]{ref}
<Alan Hodgkin>'s later account describes advancing a capillary <electrode> from a cut end into intact <axon>, avoiding scraping the <cell membrane>. Its Figure 3 uses a seawater-filled capillary internally, with external seawater as zero: the readout is internal relative to external <electric potential>. This supports a voltage across the surface <cell membrane>. As a separate ionic control, the account reports that raising external <potassium> toward its internal <concentration> reversibly abolished resting voltage; this was later work, not a control asserted for the 1939 recording. Injury and electrode-solution junction potentials limit interpretation. Voltage recording alone does not identify every conducting <ion>. A <voltage clamp> additionally uses feedback to impose voltage and measures the required <electric current>.<The ionic basis of nervous conduction by Alan Hodgkin>[Alan Lloyd Hodgkin, The ionic basis of nervous conduction, Nobel Lecture (11 December 1963), pp. 34–36, Figures 2–3: placement, seawater reference and later potassium control; p. 42: voltage clamp]{ref}
In the schematic, connect the intracellular <electrode> to the positive input and the external <reference electrode> to the negative input of a voltage recorder. After electrode-offset correction, a negative reading means $\phi_{\mathrm{inside}}<\phi_{\mathrm{outside}}$. A useful baseline check is to compare both <electrodes> in the same external <chemical solution> before insertion; this is a measurement check, not an additional historical observation claimed here. Swapping inputs reverses the readout sign without changing the <biological cell>'s <membrane potential>.
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