A therm/o/meter is an instrument for measuring temperature. In this word, what is -meter?
thermos (meaning "hot") + metron (meaning "measure").
This question relates to the etymology of thermometer.
According to the internet, thermometer is a term from the French termomètre. Coined by Jesuit Father: Jean Leuréchon in the 1630s, his inspiration came from the Greek prefix, and French suffix:
- thermos meaning "hot".
- metron meaning "measure".
You can find more of the history of the thermometer by simply doing a quick google search.
Hope this helps :)