Dr. Alfred Nier was an American physicist well-known for his work on spectrometry. Nier designed the mass spectrometers used for Manhattan Project experiments and his instruments were sent to all of the major Project sites. With his mass spectrometer, Nier helped prove that that U-235 was fissile, not the more abundant isotope U-238. Nier worked for the Kellex Corporation to design and construct the apparatuses used to monitor the separation of Uranium-235 and Uranium-238, as well as leak detectors for the K-25 gaseous diffusion plant. In this interview, Nier describes in detail his instrumentation at the University of Minnesota and his work leading up to the Manhattan Project.