Max Planck discovered the value of Planck's constant, denoted as "h," during his work on blackbody radiation in the late 19th and early 20th centuries. Planck was trying to explain the observed spectrum of radiation emitted by a blackbody, which classical physics failed to account for. To address this discrepancy, Planck proposed that the energy of electromagnetic radiation is quantized, meaning it can only occur in discrete amounts or packets.
During his investigations, Planck derived a formula that described the energy distribution of blackbody radiation. To fit the experimental data, he introduced a fundamental constant, which later became known as Planck's constant. Planck's constant relates the energy of a photon to its frequency and is a fundamental constant of nature in quantum mechanics.
The exact value of Planck's constant, h, was not initially determined by Planck but was rather established through subsequent experimental work. Planck's original work provided a theoretical foundation for the concept of quantization and the role of Planck's constant, but the precise value of h was determined through various experiments, such as the photoelectric effect and Compton scattering, conducted by other physicists in the following years. Today, the value of Planck's constant is known to be approximately 6.62607015 x 10^-34 joule-seconds.