A problem when Heisenberg tried to defend his PhD thesis

Once back in Munich, Heisenberg hastened to complete his doctoral dissertation on fluid turbulence, a topic that Sommerfeld had imposed on him to broaden his knowledge and understanding of physics. However, during the defense, his inability to answer simple questions of experimental physics, such as the separating power of a telescope, nearly cost him his doctorate. Wilhelm Wien, the professor of experimental physics, was dismayed when Heisenberg struggled to explain the operation of an electric battery. Wien wanted to recalibrate this up-and-coming theorist, but accepted the compromise proposed by Sommerfeld: Heisenberg would have his doctorate, but with passable mention. 

Reference: https://history-quantum-physics.blogspot.com/2018/12/quantum-einstein-bohr-and-great-debate.html