The quantity _h_ is independent, not only of the wave-length, but also of the temperature and nature of the emitting body. This constant, the so-called _Planck constant_, is thus a universal constant. If one uses the “absolute” units of length, mass and time (see table, p. 210), its value comes out as 6·54 × 10⁻²⁷. For the frequency 750 × 10¹² vibrations per second, corresponding to the extreme violet in the visible spectrum, the Planck energy quantum thus becomes about 5 × 10⁻¹² erg, or 3·69 × 10⁻¹⁹ foot-pounds (note that the “erg” is the “absolute” unit of work, or the amount of work done when a body is moved through a distance of 1 cm. by a force of one dyne acting in the direction of the motion, while the “foot-pound” is the work done when a force of 1 pound moves a body 1 foot in the direction of the force). For light belonging to the red end of the spectrum, the energy is about half as great. If we pass, however, to the highest frequencies and the shortest wave-lengths which are known, namely those corresponding to the “hardest” (_i.e._ most penetrating) γ-rays (see p. 78), we meet with energy quanta which are a million times larger, _i.e._ 2 × 10⁻⁶ erg, although they are still very small compared to any amount of energy measurable mechanically. This remarkable theory of quanta, which in the hands of Planck still possessed a rather abstract character, proved under Einstein’s ingenious treatment to have the greatest significance in many problems which, like heat radiation, had provided physicists with many difficulties. For by assuming that energy in general could only be given up and taken in in quanta, certain facts about the specific heats of bodies could be accounted for—facts which the older physics had proved powerless to explain. The Planck energy quanta, as Einstein showed, could also explain in a very direct and satisfactory way the _photoelectric effect_, as it is called. This effect consists in the freeing of electrons from a metal plate which ultra-violet rays are allowed to strike. The maximum velocity with which these electrons are propelled from the plate is found to be independent of the intensity of the incident light, but dependent simply on the frequency of the radiation. Careful measurements have indeed shown, as Einstein predicted, that the incident light really does utilize an energy quantum _h_ν to free each electron and give it velocity (cf. p. 172). Of the different methods which nowadays are at hand, the photoelectric effect constitutes one of the best means of determining the value of _h_. It has been applied for that purpose by Millikan, to whose ingenious experiments the most accurate direct determination of _h_ is actually due.