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Incoming links: The ionic basis of nervous conduction by Alan Hodgkin
By Alan Hodgkin and Andrew Huxley, printed as A. L. Hodgkin and A. F. Huxley. Published in Nature 144, 710–711, 21 October 1939.
Publisher record: www.nature.com/articles/144710a0
Cited passage: abstract, intracellular recording in Loligo forbesi giant axons. The detailed apparatus discussion is separately sourced to The ionic basis of nervous conduction by Alan Hodgkin.
In 1939, Alan Hodgkin and Andrew Huxley reported intracellular action potential recordings from squid giant axons at Plymouth.[ref]
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
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.[ref]
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
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 . 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.