<Tr> <Td> 6999197326970000000 ♠ 0.197 326 97 </Td> <Td> eV ⋅ μm </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> E ⋅ l </Td> <Td> </Td> </Tr> <P> The Planck constant (denoted h, also called Planck's constant) is a physical constant that is the quantum of action, central in quantum mechanics . </P> <P> First recognized in 1900 by Max Planck, it was conceived as the proportionality constant between the minimal increment of energy, E, of a hypothetical electrically charged oscillator in a cavity that contained black body radiation, and the frequency, f, of its associated electromagnetic wave . In 1905, the value E, the minimal energy increment of a hypothetical oscillator, was theoretically associated by Albert Einstein with a "quantum" or minimal element of the energy of the electromagnetic wave itself . The light quantum behaved in some respects as an electrically neutral particle, as opposed to an electromagnetic wave . It was eventually called a photon . </P>

What are the units of h in the equation e = hf