Consists of letters received, letterbooks (10 v.), notebooks, charts and graphs, formulas, manuscripts, lecture notes, a bibliography, business records, genealogical notes, speeches, and an autobiography (269 p.), dating from 1869 to 1935. The collection documents Barus's research for the United State Geological Survey, the Smithsonian Institution, and Brown University; his experiments and instruments, and scholarly discourse with other physicists. Correspondents include Edward Salisbury Dana, Ren ̌de Saussure, Charles-Edouard Guillaume, Clarence King, Friedrich Wilhelm Georg Kohlausch, Samuel P. Langley, John Wesley Powell, Ogden N. Rood, Rossiter W. Raymond, Cenek Strouhal, Arthur Gordon Webster, and Eilhard Ernst Gustav Wiedemann. Numerous manuscripts and a number of letters contain analyses or discussions of experiments. Letters from de Saussure describe formulas for the resistance of air, particularly as applied to the rate at which an airplane falls. Letters from Langley discuss various experiments including a number in aerodynamics; the assembly of an aerodrome and a "railroad" cast-off for the aerodrome; thermodynamics; and physics apparatuses; as well as Barus's overall responsibilities at the Smithsonian Institution. Webster's letters are primarily responses and suggestions for changes to Barus's experiments on interferential induction balance and vectors.

Rood recommended Barus to Clarence King for the position of physicist to the United States Geological Survey and was involved in providing the opportunity for Barus to be a candidate for a position at Brown University. Rood's letters describe his connections with Brown University faculty and trustees. Experiments with ions and nuclei are discussed in manuscripts and reviews of experiments. Correspondence for the planning of the International Congress of Radiology and Electricity in Brussels in 1909 is in the collection.