New test method for thermal conductivities issued by A. S. T. M.

Designed to provide a simple and convenient means for determining the thermal conductivity of electrical insulating materials, as well as for use in testing laboratories which are now specially equipped for heat transfer work, a new Method of Test for Comparing the Thermal Conductivities of Solid Electrical Insulating Materials has been issued by the American Society for Testing Materials. The method is the thermal analogy of the potentiometer method of comparing electrical resistances. A specimen of unknown conductivity is placed in series with a standard specimen of known conductivity between plates which are maintained at different temperatures. The respective temperature differences across the standard and the test specimens when a steady state of heat flow is attained are inversely proportional to their relative conductivities. The conductivity of the standard specimens must, of course, be measured by an absolute method. Sets of thermal conductivity standards covering the usual range of electrical insulating materials may be prepared by incorporating suitable proportions of graphite in rubber. Such sets may be calibrated in laboratories equipped to make absolute measurements of thermal conductivity.