Einstein's Viscous Advice Flowed Freely Nonetheless

This talk will examine a 1930 letter from Einstein to a medicinal chemistry assistant in Leiden, whose full name isn't identified in the text. I have ascertained that the letter was likely addressed to a student named Jan Lens who was writing his doctoral thesis in Utrecht on the properties of lyophilic colloids. I show how the letter refers to the Einstein relation for viscosity that first appeared in erroneous form in 1906 and corrected form in 1911. In the letter, Einstein alludes to the possibility of an extension of his formula to charged particles. It offers an interesting example of Einstein's free- flowing generosity in offering advice to students.