During x-ray generation, surface electrons are released from a heated cathode by thermionic emission. Their outputs are not functions of temperature differences as are thermoelectric and thermionic converters. Electrons are emitted from the barium oxide impregnated insert by thermionic emission. In 1853, Becquerel discovered thermionic emission. Tunneling and thermionic emission lifetimes both depend on having a low effective barrier height. Carrier lifetime for escape is determined by tunneling and thermionic emission lifetimes. This can be accomplished by e.g. thermionic emission or arc discharge. The other relies on vacuum tubes and the principles of thermionic emission. Some of their subsequent experiments on thermionic emission provided experimental validation for the Sommerfeld theory. A variety of effects can be harnessed to produce thermoelectricity, thermionic emission, pyroelectricity and piezoelectricity. Vacuum tubes (Thermionic valves) were among the earliest electronic components. This type of circuit was used for thermionic valve (vacuum tube) technology. One classification of thermionic vacuum tubes is by the number of active electrodes. Thermionic currents can be increased by decreasing the work function.