An anode is an electrode through which positive electric charge flows into a polarized electrical device. The flow of charge is an electric current. A common mnemonic is ACID for "Anode Current Into Device"; the key notion is that positive conventional current flows into the device; positive charges are entering the device through the anode. There is a complication because a current in metals is carried by moving electrons, and the charge on an electron is negative. Positive charge flowing into the anode is the same as (negative) electrons flowing out of the anode. Consequently, the direction of electric current is opposite to the direction of electron flow: electrons flow out of the anode to the outside circuit.A widespread misconception is that anode polarity is always positive (+). This is often incorrectly inferred from the correct fact that in all electrochemical devices, negatively charged anions move towards the anode. Actually, by the above universal definition of electric current direction, anode polarity depends on the device type and, sometimes, even on its operating mode. Consequently, as can be seen from the following examples, the anode is positive in a device that consumes power, and the anode is negative in a device that provides power: In a discharging battery or galvanic cell (diagram at right), the anode is the negative terminal because it is where the current flows into "the device" (i.e. the battery cell). This inward current is carried externally by electrons moving outwards, negative charge moving one way constituting positive current flowing the other way. In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging. In a diode, the anode is the positive terminal at the tail of the arrow symbol (base of the triangle), where current flows into 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, the anode is the positive terminal where electrons flow out of the device, i.e., where positive electric current flows in.An electrode through which current flows the other way (out of the device) is termed a cathode.
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