When was the schwa character first used by phonologists? Can it be traced to a particular author?

I believe Jacob Grimm was the first to characterize the schwa sound and identify it as a vowel. So that would give us a terminus post quem of the early nineteenth century. The first citation for schwa in the OED is from Peter Lang’s 1895 A Short Manual of Comparative Philology for Classical Students, and that uses the symbol ə, so 1895 is the terminus ante quem.

From French Wikipedia:

Johann Andreas Schmeller est le premier à utiliser le e culbuté pour représenter le son du schwa, dans son livre Die Mundarten Bayerns grammatisch dergestellt en 1821.

Thanks, you’re both champs. MUCH earlier than I’d’ve guessed.