t piec o tu Where desired, the tional practice. tubing i may be coated inside thereof with a rel- My method i t of steps Such as th fo1 atively heavy oxide insulating layer and in cases lowing: A tubular member of metallic material, Where aluminum s emp y d in fabricating this 49 such as copper, aluminum, brass and the like, is tube this Coating y be applied chemically or filled with material such