MLA

Gibbs, J. Willard (Josiah Willard), 1839-1903. Planck, Karl Ernst Ludwig (1858-1947) - Autograph letter, signed, to [Josiah Willard Gibbs]. Berlin.. Box 3, folder 104. 1902 March 29. https://collections.library.yale.edu/catalog/2020781.

APA, 6th edition

Gibbs, J. Willard (Josiah Willard), 1839-1903. (["Box 3, folder 104"])(1902 March 29)[Planck, Karl Ernst Ludwig (1858-1947) - Autograph letter, signed, to [Josiah Willard Gibbs]. Berlin.]. https://collections.library.yale.edu/catalog/2020781.