An anode is an electrode through which the conventional current enters into a polarized electrical device. This contrasts with a cathode, an electrode through which conventional current leaves an electrical device. A common mnemonic is ACID, for 'anode current into device'. The direction of conventional current (the flow of positive charges) in a circuit is opposite to the direction of electron flow, so (negatively charged) electrons flow out the anode into the outside circuit. In a galvanic cell, the anode is the electrode at which the oxidation reaction occurs. An anode is also the wire or plate having excess positive charge. Consequently, anions will tend to move towards the anode. The terms anode and cathode are not defined by the voltage polarity of electrodes but the direction of current through the electrode. An anode is an electrode through which conventional current (positive charge) flows into the device from the external circuit, while a cathode is an electrode through which conventional current flows out of the device. If the current through the electrodes reverses direction, as occurs for example in a rechargeable battery when it is being charged, the naming of the electrodes as anode and cathode is reversed. Conventional current depends not only on the direction the charge carriers move, but also the carriers' electric charge. The currents outside the device are usually carried by electrons in a metal conductor. Since electrons have a negative charge, the direction of electron flow is opposite to the direction of conventional current. Consequently, electrons leave the device through the anode and enter the device through the cathode. The definition of anode and cathode is slightly different for electrical devices such as diodes and vacuum tubes where the electrode naming is fixed and does not depend on the actual charge flow (current). These devices usually allow substantial current flow in one direction but negligible current in the other direction. Therefore the electrodes are named based on the direction of this 'forward' current. In a diode the anode is the terminal through which current enters and the cathode is the terminal through which current leaves, when the diode is forward biased. The names of the electrodes do not change in cases where reverse current flows through the device. Similarly, in a vacuum tube only one electrode can emit electrons into the evacuated tube due to being heated by a filament, so electrons can only enter the device from the external circuit through the heated electrode. Therefore this electrode is permanently named the cathode, and the electrode through which the electrons exit the tube is named the anode. The polarity of voltage on an anode with respect to an associated cathode varies depending on the device type and on its operating mode. In the following examples, the anode is negative in a device that provides power, and positive in a device that consumes power: In a discharging battery or galvanic cell (diagram at right), the anode is the negative terminal because it is where conventional current flows into 'the device' (i.e. the battery cell). This inward current is carried externally by electrons moving outwards, negative charge flowing in one direction being electrically equivalent to positive charge flowing in the opposite direction. 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.