electrode–skin interface
Between the metal of an electrode and the salty fluid of your skin sits a messy little electrochemical world — and it can ruin a recording before the amplifier ever sees the signal. Where metal meets electrolyte, charge stops being carried by electrons and starts being carried by ions; that handoff sets up a small DC 'half-cell' voltage and a frequency-dependent impedance, as if a battery and a leaky capacitor were quietly wired in series with your biopotential. Think of trying to listen to a faint radio station through a crackly, corroded antenna connector: the interface, not the broadcast, is often what you're fighting.
Two numbers dominate. First, the offset potential: a silver/silver-chloride (Ag/AgCl) wet electrode settles to a stable few-millivolt DC offset, which is why it's the clinical gold standard — bare stainless steel can drift by hundreds of millivolts and swamp a 1 mV ECG. Second, the contact impedance: good gelled electrodes sit around a few kΩ, dry or poorly-attached ones soar past hundreds of kΩ, and because that impedance forms a divider with the amplifier's input impedance, any mismatch between two electrodes converts harmless common-mode mains hum into a real differential error. The fix is brute-force high input impedance (GΩ) and obsessively matched, low-impedance contacts.
Gelled Ag/AgCl beats dry contacts on both offset stability and impedance — the two things that wreck biopotential recordings.
This is why a nurse scrubs and gels your skin before an ECG: abrading the dead outer skin layer and adding conductive gel can drop the contact impedance from hundreds of kΩ to a few kΩ, turning a noisy trace into a clean one.