cell) is called the cathode and the electrode with a negative polarity (zinc in a dry cell) is called the anode. This is the reverse of the terminology used in an electrolytic cell or thermionic vacuum tube. The reason is that the terms anode and cathode are defined by the direction of electric current, not by their voltage. The anode is the terminal through which conventional current (positive charge) enters the cell from the external circuit, while the cathode is the terminal through which conventional current leaves the cell and flows into the external circuit. Since a battery is a power source which provides the voltage which forces the current through the external circuit, the voltage on the cathode must be higher than the voltage on the anode, creating an electric field directed from cathode to anode, to force the positive charge out of the cathode through the resistance of the external circuit. Inside the cell the anode is the electrode where chemical oxidation occurs, as it donates electrons which flow out of it into the external circuit. The cathode is the electrode where chemical reduction occurs, as it accepts electrons from the circuit. Outside the cell, different terminology is used. As the anode donates positive charge to the electrolyte (thus remaining with an excess of electrons that it will donate to the circuit), it becomes negatively charged and is therefore connected to the terminal marked "−" on the outside of the cell. The cathode, meanwhile, donates negative charge to the electrolyte, so it becomes positively charged (which allows it to accept electrons from the circuit) and is therefore connected to the terminal marked "+" on the outside of the cell. Old textbooks sometimes contain different terminology that can cause confusion to modern readers. For example, a 1911 textbook by Ayrton and Mather describes the electrodes as the "positive plate" and "negative plate" . See also History of the battery Fuel cell Battery recycling List of battery sizes List of battery types Comparison of battery types Battery nomenclature References External links Nonrechargeable batteries Battery types Battery (electricity) it:Pila (chimica)