Kamerlingh Onnes led the world in low-temperature physics and discovered superconductivity. Bunsen and Kirchhoff taught him, while his close friend van der Waals’ theoretical work strongly supported his own practical research. Lorentz, a distant cousin, was also a close colleague at Leiden, where Ehrenfest also joined them. Dewar, Bohr and Langevin were all correspondents. The young Einstein wrote to Kamerlingh Onnes about a studentship, but got no reply; they later forged a close connection, Einstein enjoying frequent visits to Leiden, though he felt that Kamerlingh Onnes lacked skill at expressing his thoughts.