A cathode is the electrode through which, by convention, electric current flows out of a polarized electrical device. In physics, conventional current is regarded as a flow of positive charge – a definition that goes back to Michael Faraday (1791–1867), one of the pioneers of electrical science. (Conventional current is thus in the opposite direction to the flow of electrons.)


Contrary to many definitions that may be seen, the polarity of a cathode can be negative or positive, depending on circumstances. In the case of a discharging cell or battery, the cathode assumes a positive polarity – the outward current being borne internally by positive ions moving from the electrolyte to the cathode and continued externally by electrons moving inward (negative charge moving one way equating to positive current flowing the other way). During charging of a secondary cell, the cathode's role is reversed since it now receives electrons from the external current source.


During electrolysis, the cathode is the electrode to which positively-charged ions (cations) are attracted. In a cathode ray tube and electron tube, the cathode is the negative terminal toward which electrons flow, i.e., where current flows out of the device.