Otto Stern's 1933 measurement of the unexpectedly large proton magnetic moment indicated to most physicists that the proton is not a point particle. At that time, many physicists modeled elementary particles as point particles, and therefore Stern's discovery initiated the speculation that the proton might be a composite particle. In this work, we show that despite being an elementary particle, the proton is an extended particle. Our work is motivated by the experimental data, which we review in section 1. By applying Occam's Razor principle, we identify a simple proton structure that explains the origin of its principal parameters. Our model uses only relativistic and electromagnetic concepts, highlighting the primary role of the electromagnetic potentials and of the magnetic flux quantum Phi(M) = (h)/(e): Unlike prior proton models, our methodology does not violate Maxwell's equation, Noether's theorem, or the Pauli exclusion principle. Considering that the proton has an anapole (toroidal) magnetic moment, we propose that the proton is a spherical shaped charge that moves at the speed of light along a path that encloses a toroidal volume. A magnetic flux quantum Phi(M) = (h)/(e) stabilizes the proton's charge trajectory. The two curvatures of the toroidal and poloidal current loops are determined by the magnetic forces associated with Phi(M). We compare our calculations against experimental data.
