Discharge lamp having fuse-switch guard against jacket failure

Claim 1

A jacketed electric lamp comprising a vitreous outer envelope having inleads sealed therethrough; a vacuum or a nonoxidizing filling in said outer envelope; an inner envelope of material which transmits ultraviolet radiation within said outer envelope; said inner envelope having electrodes sealed into its ends and containing an ionizable medium comprising a metal which produces radiation including ultraviolet which is transmitted by the inner envelope and normally intercepted at the outer envelope; and means connecting the electrodes of the inner envelope to the inleads of the outer envelope, said means including a fuse-switch comprising a resistive heater connected in series circuit with the inner envelope and made of a metal that oxidizes rapidly upon contact with air at the temperature caused by normal lamp current flow through it, and a thermal switch shunting said heater and located in heat-receiving relationship from said heater and from said inner envelope, said switch being normally open when the lamp is off but closing upon heating by said heater, and thereafter being held closed by heat from said inner envelope when the lamp is operating normally but cooling and reopening upon air contact should the outer envelope be broken, whereupon said heater carries lamp current and oxidizes rapidly, culminating in burn through of said heater and permanent disablement of the lamp.