Experiments using a simple thermal comparator for measurement of thermal conductivity, surface roughness and thickness of foils or of surface deposits

Abstract

The thermal comparator described consists of two metal balls similarly mounted in a block of balsa wood, but one is at a slightly lower level so that it touches any surface on which the block rests. After heating to a small fixed temperature excess the block is laid in contact with the test surface. Differentially connected thermocouples attached to each ball measure the increased rate of cooling of the ball which makes contact. The differential e.m.f. is observed 10 s after contact is made and is shown to be a function of the thermal conductivity of the material on which the ball rests. When calibrated in terms of at least two substances of known thermal conductivity, determinations of thermal conductivity can be quickly made. The accuracy from single readings made on several alloys of iron, aluminium and magnesium is about ±6%, but lead departs considerably, due, it is thought, to its softness. The condition of the surface affects the readings, so the calibration should be made for a similar surface condition. Alternatively, the device can be used as a means of measuring surface finish. Other possible uses are for measuring the thickness of surface deposits and for non-destructive sorting or identification of solid materials.