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A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition is sometimes remembered using the mnemonic CCD for ''cathode current departs''. A conventional current describes the direction in which positive electronic charges move. Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic ''cathode current departs'' also means that electrons flow into the device's cathode. Cathode polarity with respect to the anode can be positive or negative; it depends on how the device operates. Although positively charged cations always move towards the cathode (hence their name) and negatively charged anions move away from it, cathode polarity depends on the device type, and can even vary according to the operating mode. In a device which takes energy (such as recharging a battery), the cathode is negative, and in a device which provides energy (such as discharging a battery), the cathode is positive: * In a discharging battery or a galvanic cell, the cathode is the positive terminal since that is where the current flows out of the device (see drawing). This outward 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, this negative charge moving inwards constituting positive current flowing outwards. For example, the Daniell galvanic cell's copper electrode is the positive terminal and the cathode. * In a recharging battery, or an electrolytic cell performing electrolysis, the cathode is the negative terminal, from which current exits the device and returns to the external generator. For example, reversing the current direction in a Daniell galvanic cell would produce an electrolytic cell,〔(), Daniell cell can be reversed to, technically, produce an electrolytic cell.〕 where the copper electrode is the positive terminal and the anode. * In a diode, the cathode 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 vacuum tubes (including cathode ray tubes) it is the negative terminal where electrons enter the device from the external circuit and proceed into the tube's near vacuum, constituting a positive current flowing 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〔Ross, S, ''Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry'' in ''Notes and Records of the Royal Society of London (1938–1996)'', Volume 16, Number 2 / 1961, Pages: 187–220, () consulted 2006-12-22〕 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".〔Faraday, Michael, ''Experimental Researches in Electricity. Seventh Series, Philosophical Transactions of the Royal Society of London (1776–1886)'', Volume 124, 1 January 1834, Page 77, () consulted 2006-12-27 (in which Faraday introduces the words ''electrode'', ''anode'', ''cathode'', ''anion'', ''cation'', ''electrolyte'', ''electrolyze'')〕〔Faraday, Michael, ''Experimental Researches in Electricity'', Volume 1, 1849, reprint of series 1 to 14, freely accessible Gutenberg.org transcript () consulted 2007-01-11〕 The use of 'West' to mean the 'out' direction (actually 'out' → 'West' → 'sunset' → 'down', i.e. 'out of view') 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 technically correct (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'. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cathode」の詳細全文を読む スポンサード リンク
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