Rapid measurement of thermal conductivity by transient heating of a fine thermo-junction
Transient methods of measuring thermal conductivities of poor conductors are very rapid and can be applied to small systems. By recording the temperature of an electrically heated wire, the conductivity of the environment can be determined in a fraction of a second. Thus with a wire of radius 10 -2 cm the measuring time is less than 10 -1 s and the effective overall radius of the system is about 10 -1 cm; furthermore, the effective length can be reduced to about 1·0 cm by making the wire a thermo-junction. Apparatus operating on this principle consists of a source of radio-frequency current for heating the junction wires and an oscillograph for simultaneous recording of the thermo-e. m. f. Temperature-time curves are used to deduce the thermal conductivity of substances placed in contact with the wires. The method is very flexible and can be applied to small, rapidly changing systems at high temperatures. The conductivities of certain liquids have been measured at room temperature. The values obtained generally lie within 10% of the accepted values.