An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron (meaning amber, from which the word electricity is derived) and hodos, a way.

Polarizable and Non-Polarizable Electrodes :

Perfectly Polarizable Electrodes :
These are electrodes in which no actual charge crosses the electrode-electrolyte interface when a current is applied. The current across the interface is a displacement current and the electrode behaves like a capacitor. Example : Ag/AgCl Electrode

Perfectly Non-Polarizable Electrode
These are electrodes where current passes freely across the electrode-electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials. Example : Platinum electrode

Commonly Used Biopotential Electrodes :

Metal plate electrodes :
–Large surface: Ancient, therefore still used, ECG
–Metal disk with stainless steel; platinum or gold coated
–EMG, EEG
–smaller diameters
–motion artifacts
–Disposable foam-pad: Cheap

Suction Cup electrodes
- No straps or adhesives required
- precordial (chest) ECG
- can only be used for short periods


Floating electrodes
- metal disk is recessed
- swimming in the electrolyte gel
- not in contact with the skin
- reduces motion artifact


Needle and wire electrodes for percutaneous measurement of biopotentials


Fetal ECG Electrodes :

Electrodes for detecting fetal electrocardiogram during labor, by means of intracutaneous needles (a) Suction electrode. (b) Cross-sectional view of suction electrode in place, showing penetration of probe through epidermis. (c) Helical electrode, which is attached to fetal skin by corkscrew type action.