center in order to maintain a more uniform temperature distribution throughout the entire volume of the furnace therefore it is preferable to space the turns of the helical groove 16 more closely in end regions 28 of ceramic tube 12 than in the central portion 30 as shown in fig2 resistance wire 18 will then be wound more tightly more turns per unit length in end regions 18 than in central portion 30 providing more heat to the end regions to achieve balance with heat loss and ensure uniform constant temperature within the furnace although as shown in fig2 the winding pitch in end regions 28 is constant and greater than the constant pitch in central region 30 in other embodiments variable winding pitches along the entire length of the ceramic tube 12 could be utilized fig3 is an end view of ceramic cylinder 12 with interior metal liner 22 showing the axial expansion slot 24 extending along the entire length of the liner and thermocouple positioning slot 26 extending partially along the length of the liner the insulating housing 14 which encloses the electrically heated ceramic tube 12 with metal liner 22 is formed of a metal can or other container 32 which has.