was born in 1800 in Neudietendorf, Saxony. He came to homeopathy through Hahnemann's pupil, Aegidi, who treated him successfully and introduced him to Hahnemann. He worked on “The Chronic Diseases” from 1825 and was given the task by Hahnemann of compiling a dictionary of symptoms. He was a great devotee of Hahnemann, following him to Paris and working with him there until Hahnemann's death.
Jahr's first major publication was his materia medica with repertory, which appeared for the first time at the beginning of the 1830s. Together with Boenninghausen, Jahr was therefore the first to index symptoms and produce a repertory. This work was frequently reissued up to the 1860s.
His second important book was the four-volume symptom codex (2 volumes materia medica and 2 volumes repertory), which was supplemented with an alphabetic repertory of skin symptoms.
While compiling the symptom codex, he published a clinical guide (in 1849 for the first time), which was reissued 3 times. His final great work appeared in 1867, the “Therapeutic Guide.” Jahr died on July 11, 1875 in Belgium.