Prague, 21 October 1911

On electrical conductivity of metals, Sommerfeld's hypothesis on collision time, an Ehrenfest paper on radiation, and measurements of compressibility and electricity.

Einstein writes on a variety of scientific subjects amidst a flurry of activity, having been away first in Karlsruhe for a meeting of the Gesellschaft Deutscher Naturforscher und Ärzte, and then in Zurich, where he has given a series of lectures on theoretical physics; and he is about to go to the Solvay Congress ('the witches' sabbath in Brussels'). He offers a critique of a formula offered by Besso for the electrical conductivity of metals, giving particular consideration to temperature variables, and moves on to discuss Sommerfeld's hypothesis about collision time (that collisions between electrons and atoms do not take place instantaneously), noting that he formulated it 'without any theory ... But one can advance really weighty grounds for it directly from the radiation formula. It is not compatible with our mechanics; it is useless to rack one's brain about it ... For low speeds, Sommerfeld's hypothesis cannot be valid – it must be some kind of yet to be understood limiting law'. Einstein likes Besso's idea about a toggle lever for measuring compressibility, and discusses Habicht's progress in constructing a 'Maschinchen' – an apparatus for the measurement of small quantities of electricity. He inserts a diagram to show the character of a curve in a radiation formula discussed in a recent paper by Ehrenfest.