The electronic heat capacity was overestimated by the 1900 Drude-Lorentz model to be half of the value predicted by Dulong–Petit. With the development of the quantum mechanical free electron model in 1927 by Arnold Sommerfeld the electronic contribution was found to be orders of magnitude smaller. This model explained why conductors and insulators have roughly the same heat capacity at large temperatures as it depends mostly on the lattice and not on the electronic properties.