A cathode is one of the two electrodes used either in a vacuum tube or in an electrochemical cell. An electrode is the part (pole) of a vacuum tube or cell through which electricity moves into or out of the system. The other pole of the system is referred to as the anode.
A vacuum tube is a hollow glass cylinder from which as much air as possible has been removed. In a vacuum tube, the cathode is the negative electrode. It has more electrons on its surface than does the other electrode, the anode. Electrons can accumulate on the surface of a cathode for various reasons. For example, in some vacuum tubes, the cathode is heated to a high temperature to remove electrons from atoms that make up the cathode. The free electrons are then able to travel from the cathode to the anode. These streams of electrons are known as cathode rays, and the tubes in which they are produced are called cathode-ray tubes (CRTs). CRTs are widely used as oscilloscopes (which measure changes in electrical voltage over time), television tubes, and computer monitors.
Electrochemical cells are devices for turning chemical energy into electrical energy or, alternatively, changing electrical energy into chemical energy. Electrochemical cells are of two types: voltaic cells (also called galvanic cells) and electrolytic cells. In a voltaic or galvanic cell, electrical energy is produced as the result of a chemical reaction between two different metals immersed in a (usually) water solution. The differing tendency of the two metals to gain and lose electrons causes an electric current to flow through an external wire connecting the two metals. The cathode is defined in a system of this type as the metal at which electrons are being taken up from the external wire. In contrast, the anode is the point at which electrons are being given up to the external wire.
Cathodes are used in many practical applications. For example, electroplating is a process by which a layer of pure metal can be deposited on a base used as the cathode. Suppose that a spoon composed of iron is made the cathode in an electrochemical cell that also contains an anode made of silver metal and a solution of silver nitrate. In this cell, silver atoms lose electrons that travel through an external circuit to the cathode. At the cathode, the electrons combine with silver ions in the solution to form silver atoms. These silver atoms plate out on the surface of the iron spoon, giving it a coating of silver metal. The plain iron spoon soon develops a shiny silver surface. It looks more attractive and is less likely to rust than the original iron spoon.