S = k log W
where k is Boltzmann's constant.
The Austrian physicist Ludwig Boltzmann first linked entropy and probability in 1877. However, the equation as shown, involving a specific constant, was first written down by Max Planck, the father of quantum mechanics in 1900. In his 1918 Nobel Prize lecture, Planck said:
This constant is often referred to as Boltzmann's constant, although, to my knowledge, Boltzmann himself never introduced it – a peculiar state of affairs, which can be explained by the fact that Boltzmann, as appears from his occasional utterances, never gave thought to the possibility of carrying out an exact measurement of the constant. Nothing can better illustrate the positive and hectic pace of progress which the art of experimenters has made over the past twenty years, than the fact that since that time, not only one, but a great number of methods have been discovered for measuring the mass of a molecule with practically the same accuracy as that attained for a planet.