Abstract: After a flexible printed circuit is left under an atmosphere of a water vapor pressure of not lower than 30 mmHg so as to absorb moisture, a voltage is applied between two wiring portions to thereby measure an insulation resistance value between the wiring portions during the voltage application. The quantity &Dgr;log(R) of the change of the insulation resistance value at regular intervals is obtained. An examination is made as to whether the maximum of the quantity &Dgr;log(R) of the change of the insulation resistance value is not larger than a predetermined reference value or not. From this examination, a judgment is made as to whether the flexible printed circuit is good or not.

Abstract: A vacuum-insulated, double-walled metal structure comprises inner and outer shells made of a metal and seamed together to form a double-walled structure with a space between them, the space being exhausted of air. At least one of the inner and outer shells is provided with at least one activated foil of a metal selected from the group consisting of copper, titanium and zirconium. The inner shell may be wound with an activated titanium or zirconium foil, and covered by another metal foil selected from the group consisting of copper, aluminum, titanium and zirconium, except for a part of its bottom. The metal foil wound on the metal structure is activated by preliminarily exhausting air from the space so that a pressure therein becomes reduced to a low vacuum of the order of 10.sup.-2 Torr, and then heating the double-walled structure at not less than about 400.degree. C.