This collection contains photographs of scientific instruments and equipment designed, built, used, or modified by the Department of Terrestrial Magnetism (DTM) at the Carnegie Institution of Washington (CIW) between 1909 and the mid-1950s. The collection includes prints, negatives, and glass plate negatives and documents the DTMs wide-ranging work on topics in geomagnetism, atmospheric electricity, radio propagation and the ionosphere, cosmic rays, atomic physics, oceanography, seismology, radio and optical astronomy, and isotope geochemistry. Many of the images depict the various instruments used by DTM during the four decades in which it conducted a global magnetic survey (1904-1946). Also well-documented is a series of particle accelerators (including Tesla coils, Van de Graaff generators, and a cyclotron) built from the 1920s through the 1940s to probe the structure of the atom. Photographs from the post-WWII era reflect the Departments new areas of investigation in seismology, isotope geology, and astronomy. Complementing these are images of magnetic observatories, physical laboratories, research vessels, and other facilities related to the Departments investigations.