Metal vapor arc lamp having thermal link diminishable in heat conduction

A method of manufacturing arc discharge lamps of the kind comprising an outer vitreous envelope having a pair of lead-in conductors sealed into it and an arc tube supported within said outer envelope and connected across said lead-in conductors, said arc tube having an ionizable medium sealed within it which includes vaporizable metal in a quantity exceeding that vaporized during lamp operation, the heat balance in the lamp determining a cold spot in said arc tube whereat excess metal collects, and the temperature of said cold spot determining the metal vapor pressure in the arc tube and the voltage drop across it, and a metal member in said lamp the heat loss from which influences significantly the temperature of said cold spot, which method comprises:

• making lamps to the extent described hereinabove and providing in each lamp a thermal link to said metal member proportioned to make lamp production consist almost entirely of lamps whose voltage drop is within specified limits together with a substantial percentage of lamps whose voltage is below the lower specified limit,
• measuring the voltage drop in completed lamps and segregating those wherein the drop is below the lower limit,
• and then reducing the heat conduction of the thermal links in the segregated lamps sufficiently to raise their voltage drop above the lower limit without breaking open the outer envelope.