The Eyring equation, another Arrhenius-like expression, appears in the " transition state theory " of chemical reactions, formulated by Eugene Wigner, Henry Eyring, Michael Polanyi and M. G. Evans in the 1930s. The Eyring equation can be written: k = k B T h e − − Δ Δ G ‡ ‡ R T = k B T h e Δ Δ S ‡ ‡ R e − − Δ Δ H ‡ ‡ R T , where Δ Δ G ‡ ‡ is the Gibbs energy of activation, Δ Δ S ‡ ‡ is the entropy of activation, Δ Δ H ‡ ‡ is the enthalpy of activation, k B is the Boltzmann constant, and h is the Planck constant.