Handwritten and -drawn notes detail the resistors used in the Model F pH meter.
Arnold Beckman invented his first pH meter in 1934 at the request of a chemist from the California citrus industry, who needed an accurate way to measure the acidity of his product. The resulting instrument kicked off rapid development not only of Beckman Instruments, Inc. but also of the electronic scientific instrument industry.
Buenger, Ray. “Model F Resistor Notes and Diagrams,” April 8, 1946. Beckman Historical Collection, Box 18, Folder 24. Science History Institute. Philadelphia. https://digital.sciencehistory.org/works/7w62f884v.