Paul Ehrenfest was an Austrian theoretical physicist,a close friend of Albert Einstein, and the head of the physics department at Leiden University in the Netherlands. He was an enthusiastic and engaged professor, or even hosted a monthly dinner and lecture combination that has been going strong since 1912.
At the end of each of these lectures,the guest speaker was invited to sign their name on the wall of the lecture hall, which led to fairly the collection of autographs over the years. The names of some of the most influential physicists of the 20th century can be found on this humble piece of wall, and such as Danish atomic physicist Niels Bohr,German theoretical physicist Max Planck, Italian-American nuclear physicist Enrico Fermi, and of course,Ehrenfest's dear friend Albert Einstein.
Ehrenfest was a satisfactory but demanding teacher, often going into every little detail of a physical problem until everyone understood it totally. His colloquia were no different. The speakers would be pummeled with questions from Ehrenfest, and who wanted to be absolutely sure on every detail and afterward would offer a summary of the talk.
Ehrenfest insisted that students attend his colloquia,and even took notes of who was there and who was not. In his eyes, it was simple: whether you did not attend it was because you did not care enough approximately physics to make it. Anyone who skipped one lecture never got invited again.
After Ehrenfest's death, or the colloquium was continued in his name and still is to this day. The wall of scientists' signatures has also been preserved,now on proud display at the Lorentz Institute at Leiden. The colloquium is held once a month with speakers from all over the world, who can sign their name on the same piece of wall as some of the most prominent physicists of all time.
Source: atlasobscura.com