Cathode

A cathode is an electrode through which (positive) electric current flows out of a polarized electrical device. Mnemonic: CCD (Cathode Current Departs). To dispel a common misconception, often incorrectly inferred from the correct fact that in all electrochemical devices positively charged cations move towards the cathode and/or negatively charged anions move away from it, cathode polarity is not always negative but depends on the device type, and sometimes even in which mode it operates, as determined by the above current direction based universal definition. Examples:
 * In a discharging battery or galvanic cell (drawing) the cathode is the positive terminal, where conventional current flows out. This outwards current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). It is continued externally by electrons moving inwards, negative charge moving one way amounting to positive current flowing the other way.
 * In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator.
 * In a diode, it is the negative terminal at the pointed end of the arrow symbol, where current flows out of the device. Note electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as zener diodes or solar cells where the current of interest is the reverse current.
 * In a cathode ray tube, it is the negative terminal where electrons flow in, i.e., where current flows out of the device.

An electrode through which current flows the other way (into the device) is termed an anode.

Etymology
The word was coined in 1834 from the Greek κάθοδος (kathodos), 'descent' or 'way down', by William Whewell, who had been consulted by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis. In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in a direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the cathode is where the current leaves the electrolyte, on the West side: "kata downwards, `odos a way ; the way which the sun sets" (, reprinted in ).

The use of 'West' to mean the 'out' direction (actually 'out' → 'West' → 'sunset' → 'down') may appear unnecessarily contrived. Previously, as related in the first reference cited above, Faraday had used the more straightforward term "exode" (the doorway where the current exits). His motivation for changing it to something meaning 'the West electrode' (other candidates had been "westode", "occiode" and "dysiode") was to make it immune to a possible later change in the direction convention for current, whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in the event of a later convention change it would have become West to East, so that the West electrode would not have been the 'way out' any more. Therefore "exode" would have become inappropriate, whereas "cathode" meaning 'West electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the cathode's function any more, but more importantly because, as we now know, the Earth's magnetic field direction on which the "cathode" term is based is subject to reversals whereas the current direction convention on which the "exode" term was based has no reason to change in the future.

Since the later discovery of the electron an easier to remember, and more durably correct technically although historically false etymology has been suggested: cathode, from the Greek kathodos, 'way down', 'the way (down) into the cell (or other device) for electrons'.

Flow of electrons
The flow of electrons is always from anode to cathode outside of the cell or device, regardless of the cell or device type and operating mode, with the exception of diodes where electrode naming always assumes current flows in the forward direction (that of the arrow symbol), i.e., electrons flow in the opposite direction, even when the diode reverse-conducts either by accident (breakdown of a normal diode) or by design (breakdown of a Zener diode, photo-current of a photodiode or solar cell).

Chemistry cathode
In chemistry, a cathode is the (negative or positive, depending on whether the cell is electrolytic or galvanic) electrode of an electrochemical cell at which reduction occurs (electrons are added to cations to complete the valence shell or bond). The cathode supplies electrons to the positively charged cations which flow to it from the electrolyte (even if the cell is galvanic, i.e., when the cathode is positive and therefore would be expected to repel the positively charged cations; this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems in a galvanic cell).

Electrolytic cell
In an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions.

Galvanic cell
In a galvanic cell, the cathode is where the positive pole is connected to allow the circuit to be completed: as the anode of the galvanic cell gives off electrons, they return from the circuit into the cell through the cathode.

Electroplating metal cathode
When metal ions are reduced from ionic solution onto the cathode, they form a pure metal surface on the cathode. Items to be plated with pure metal are attached to and become part of the cathode in the electrolytic solution.

Electronics and physics cathode
In physics or electronics, a cathode is an electrode that emits electrons into the device.

Vacuum tubes
In a vacuum tube or electronic vacuum system, the cathode emits free electrons. Electrons are extracted from metal electrodes either by heating the electrode, causing thermionic emission, or by applying a strong electric field and causing field emission. Electrons can also be emitted from the electrodes of certain metals when light of frequency greater than the threshold frequency falls on it. This effect is called photoelectric emission.

Cold cathodes and hot cathodes
Cathodes used for field emission in vacuum tubes are called cold cathodes. Heated electrodes or hot cathodes, frequently called filaments, are much more common. Most radios and television sets prior to the 1970s used filament-heated-cathode electron tubes for signal selection and processing; to this day, a hot cathode forms the source of the electron beam(s) in cathode ray tubes in many television sets and computer monitors. Hot electron emitters are also used as the electrodes in fluorescent lamps and in the source tubes of X-ray machines.

Diodes
In a semiconductor diode, the cathode is the N–doped layer of the PN junction. Initially, the N-doped layer supplies electrons to flow into the junction (N-doped for negative charge carriers). The electrons given by the N-doped layer combine with 'holes' supplied from the P-doped layer. The electrons and holes combining create a 'depleted' zone at the junction, leaving behind in the cathode a thin layer of positive ions which gives a base positive charge to the junction's cathode side of the device. (The anode side has a base negative charge at the junction, since it supplied 'holes' to the recombinant region and the doped ions have one electron more than their full valence shell of electrons). As a negative charge is applied to the cathode from the circuit external to the diode, more N-doped ions are able to supply electrons to the recombinant region and the diode becomes conductive, which allows electrons to flow though the diode from the cathode to the anode (electrons flow from the N-doped to the P-doped side when the bias is overcome). Like a typical diode, there is a fixed anode and cathode in a zener diode, but it will conduct current in the reverse direction (electrons from anode to cathode) if its breakdown or Zener voltage is exceeded.