Electromagnetic Separation

The technique of electromagnetic separation is based on the same principles as mass spectrometry. When charged particles, such as uranium nuclei, are accelerated through a magnetic field, their trajectories are shaped by this field. Lighter particles are deflected farther from the straight-line path than heavier particles, and the path of motion of a lighter particle is thus much more curved than is the path of the heavier particle. The slight mass difference between uranium-235 and uranium-238 means that the two isotopes are affected differently by the magnetic field. The lighter isotope, uranium-235, therefore, can be separated physically from the heavier uranium-238 into different collecting slots built into the machine.

https://www.osti.gov/opennet/manhattan-project-history/Processes/UraniumSeparation/electromagnetic.html