How was the Planck's constant calculated ?

Question

They must have used all the equipments of the 20th century, by which, the answer for such a small constant must have come as good as zero? So how was it done ?

Answer 1

Robert A. Millikan 1916
" Millikan was the first to determine with great accuracy that the maximum kinetic energy of the ejected electrons obey the equation Einstein had proposed in 1905: namely, 1/2mv2 = hf - P, where h is Planck's constant, f the frequency of the incident light, and P is, in Millikan's words, "the work necessary to get the electron out of the metal." Millikan determined h to have the value 6.57 x 10-27 erg-sec to "a precision of about 0.5 per cent," a value far better than had been obtained in any previous attempt."
http://focus.aps.org/story/v3/st23