1. David Kaiser, “The Origins of Physics Today,” Physics Today 71, no. 5 (2018), 32–38, on 34.
2. David Kaiser, “Booms, Busts, and the World of Ideas: Enrollment Pressures and the Challenge of Specialization,” Osiris 27 (2012), 276–302, on 290, 391, figure 6.
3. Kaiser, “Booms, Busts” (ref. 2), 299, figure 7.
4. Kaiser, “Booms, Busts” (ref. 2) 294–95. For the current classification system used by the APS and in INSPEC, Section A, Physics (the successor to Physics Abstracts), see https://publishing.aip.org/publishing/pacs/pacs-2010-regular-edition and https://www.theiet.org/resources/inspec/about/coverage/ .
5. Kaiser, “Booms, Busts” (ref. 2), 296–97.