Files contain notes for lectures on such topics as: heat; optics; acoustics; magnetism; 'galvanometry'; and experimental physics. Writings on: Roentgen's discovery of x-rays; the Toepler induction machine, etc. Personal files, awards; memberships. There are 200-300 correspondents in the collection, including family members, particularly his son. There is scientific correspondence, but also personal correspondence with scientists including C. Rn̲tgen, H. V. Helmholz, Ludwig Boltzmann, A. Ettinghausen, Gustav Zeuner, Hallwachs, Heinrich Hertz, Werner Siemens, and others. Also correspondnece with scientific organizations and firms, for example, academies, and Carl Zeiss of Jena. Also correspondence with government offices and the royal house.