before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from Greek thermos "hot, warm," therme "heat" (from PIE root *gwher- "to heat, warm").
modern word-forming element meaning "instrument for recording; that which writes, marks, or describes; something written," from Greek -graphos "-writing, -writer" (as in autographos "written with one's own hand"), from graphe "writing, the art of writing, a writing," from graphein "to write, express by written characters," earlier "to draw, represent by lines drawn" (see -graphy). Adopted widely (Dutch -graaf, German -graph, French -graphe, Spanish -grafo). Related: -grapher; -graphic; -graphical.
<a href="https://www.etymonline.com/word/thermograph">Etymology of thermograph by etymonline</a>
Harper, D. (n.d.). Etymology of thermograph. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thermograph