Method for testing panel-to-funnel sealing layer of a cathode-ray tube

In the manufacture of a cathode-ray tube of the type comprising (i) an evacuated envelope including an electrically-insulating glass faceplate panel having an integral peripheral sidewall and an electrically-insulating funnel hermetically sealed to said sidewall with a sealing layer of an electrically-insulating substance, (ii) an electrically-conducting coating on the internal surface of said funnel, and (iii) an electrically-conducting metal structure around the outside of said sidewall, a method for testing said sealing layer comprising, prior to connecting said tube with its ultimate circuitry for functional use, wherein a high operating DC voltage is applied to said internal funnel coating, the step of applying a high testing DC voltage between said metal structure and said internal funnel coating, said testing DC voltage being substantially higher than said operating DC voltage, whereby, if said sealing layer has insufficient resistance to electrical breakdown, said sealing layer is punctured by a leakage current and said envelope is devacuated through said puncture.