A quantity with the dimensions of energy which determines the thermionic emission of a solid at a given temperature. The thermionic electron current density J emitted by the surface of a hot conductor at a temperature T is given by the Richardson-Dushman formula, J = AT²e^−φ/kT, where A is a constant, k is Boltzmann's constant (= 1.38 × 10⁻²³ joule per degree Celsius) and φ is the work function; the last may be determined from a plot of log (J/T²) versus 1/T. For metals, φ may also be determined by measuring the photoemission as a function of the frequency of the incident electromagnetic radiation; φ is then equal to the minimum (threshold) frequency for which electron emission is observed times Planck's constant h (= 6.63 × 10⁻ joule second). The work function of a solid is usually expressed in electronvolts (1 eV is the energy gained by an electron as it passes through a potential difference of 1 V, and is equal to 1.60 × 10⁻¹⁹ J). A list of average values of work functions (in electronvolts) for metals is given in the table.