diathermometern.
Etymology: < Greek διά through + θερμόν heat + -meter comb. form2.Previous versions of the OED give the stress as: diatherˈmometer.
1883 Diathermometer, an instrument designed to measure the thermal resistance of a body by registering the amount of transmitted heat.
This entry has not yet been fully updated (first published 1895; most recently modified version published online June 2019).