However, as with so many people, he was influenced by an excellent school teacher. In his last couple of years at school his physics teacher strongly encouraged him, gave him texts on the differential and integral calculus and suggested that he read undergraduate texts on mathematics and physics. One of the books which made him enthusiastic about physics was Lorentz's Lectures on Physics which he read in the Royal Library in The Hague.
Uhlenbeck was then admitted by Ehrenfest (a student of Boltzmann's) to the Wednesday evening physics colloquium in Leiden. Ehrenfest became the most important scientific influence in his life. From 1922 to 1925 Uhlenbeck was the tutor of the younger son of the Dutch ambassador in Rome. While there, he attended lectures by Tullio Levi-Civita and Vito Volterra.
He returned to Leiden in 1925 to become Ehrenfest's assistant. Ehrenfest assigned him to work with his graduate student, Samuel Goudsmit for a quick update on "what was currently happening in physics". In mid-September 1925, Uhlenbeck and Goudsmit discovered the spin on the electron. In 1927 Uhlenbeck earned his Ph.D. degree under Ehrenfest with his thesis titled: "Over Statistische Methoden in de Theorie der Quanta" ("On Statistical Methods in the Quantum Theory" ).
He received a doctorate from the Leiden University in 1927. As a graduate student in 1925, he and Samuel Goudsmit introduced the concept of electron spin, which posits an intrinsic angular momentum for all electrons.