a high intensity discharge lamp with a safety switch which automatically shuts off the lamp upon rupture of the outer envelope . the switch is formed by a carbon rod disposed in the interior space between the arc tube and the outer envelope and connected in series with one of the electrodes so that the current flow through the rod maintains it at an elevated temperature during operation of the lamp . upon rupture of the outer envelope , the carbon rod oxidizes and falls , thereby interrupting the electrical path to the electrodes .