▁technique ▁is ▁based ▁on ▁recording ▁the ▁trans ient ▁temperature ▁rise ▁of ▁a ▁thin ▁vertical ▁metal ▁wire ▁with ▁infinite ▁length ▁when ▁a ▁step ▁voltage ▁is ▁applied ▁to ▁it . ▁The ▁wire ▁is ▁imm ers ed ▁in ▁a ▁fluid ▁and ▁can ▁act ▁both ▁as ▁an ▁elect rical ▁he ating ▁element ▁and ▁a ▁resistance ▁therm ometer . ▁The ▁trans ient ▁hot ▁wire ▁method ▁has ▁advantage ▁over ▁the ▁other ▁thermal ▁conduct ivity ▁methods , ▁since ▁there ▁is ▁a ▁fully ▁developed ▁theory ▁and ▁there ▁is ▁no ▁cal ibration ▁or ▁single - point ▁cal ibration . ▁Furthermore , ▁because ▁of ▁the ▁very ▁small ▁meas uring ▁time ▁( 1 ▁s ) ▁there ▁is ▁no ▁con ve ction ▁present ▁in ▁the ▁measurements ▁and ▁only ▁the ▁thermal ▁conduct ivity ▁of ▁the ▁fluid ▁is ▁measured ▁with ▁very ▁high ▁accuracy . ▁ ▁The ▁most ▁of ▁the ▁trans ient ▁hot ▁wire ▁sens ors ▁used ▁in ▁academ ia ▁consist ▁of ▁two ▁identical ▁very ▁thin ▁w ires ▁with ▁only ▁difference ▁in ▁the ▁length . ▁S ens ors ▁using ▁a ▁single ▁wire ▁are ▁used ▁both ▁in ▁academ ia ▁and ▁industry ▁with ▁the ▁advantage ▁over ▁the ▁two - wire ▁sens ors ▁in ▁the ▁ease ▁of ▁handling ▁of ▁the ▁sensor ▁and ▁change ▁of ▁the ▁wire . ▁ ▁An ▁A ST M ▁standard ▁is ▁published ▁for ▁the ▁measurements ▁of ▁engine ▁cool ants ▁using ▁a ▁single - trans ient ▁hot ▁wire ▁method . ▁ ▁History ▁▁▁ 2 0 0 ▁years ▁ago ▁scient ists ▁were ▁using ▁a ▁cru de ▁version ▁of ▁this ▁method ▁to ▁make ▁the ▁first ▁ever ▁thermal ▁conduct ivity ▁measurements ▁on