<Li> In metals, the charge carriers are electrons . One or two of the valence electrons from each atom is able to move about freely within the crystal structure of the metal . The free electrons are referred to as conduction electrons, and the cloud of free electrons is called a Fermi gas . </Li> <Li> In electrolytes, such as salt water, the charge carriers are ions, which are atoms or molecules that have gained or lost electrons so they are electrically charged . Atoms that have gained electrons so they are negatively charged are called anions, atoms that have lost electrons so they are positively charged are called cations . Cations and anions of the dissociated liquid also serve as charge carriers in melted ionic solids (see e.g. the Hall--Héroult process for an example of electrolysis of a melted ionic solid). Proton conductors are electrolytic conductors employing positive hydrogen ions as carriers . </Li> <Li> In a plasma, an electrically charged gas which is found in electric arcs through air, neon signs, and the sun and stars, the electrons and cations of ionized gas act as charge carriers . </Li> <Li> In a vacuum, free electrons can act as charge carriers . In the electronic component known as the vacuum tube (also called valve), the mobile electron cloud is generated by a heated metal cathode, by a process called thermionic emission . When an electric field is applied strong enough to draw the electrons into a beam, this may be referred to as a cathode ray, and is the basis of the cathode ray tube display widely used in televisions and computer monitors until the 2000s . </Li>

The charge carriers in ionic ceramics can be