Hot-Wire Instruments

Hot-Wire Instruments Points : Hot-Wire Instruments, Definition, Materials Hot-wire instruments (ammeters) were very popular in the last decade of the nineteenth century and in the first two decades of the twentieth century but because of their inherent draw-backs they have been largely superseded by thermo-couple instruments and are not used extensively now-a-a- days. However due to their good transfer characteristics at high frequencies, they are employed for determination of the differences of indications of other types of instruments on a.c. and d.c. and in special measurement applications. Materials Early hot-wire instruments were made with platinum - silver allay wire. These had a serious defect that their full scale operating temperature was low (about 150°C). The adoption of platinum- iridium alloy wire, which would be safely operated at temperatures up to 300 °C. This made an exact match of expansion coefficient of the wire and its mounting frame less important and largely eliminated this defect.