Shown are 6 different brands of US made mercury vapor lamps Lamp at the top of picture is made by Radiant, 400 watt, H33-1CD. Lamp at left is made by Sylvania, 175 watt, H39-22KB, girder frame construction. Next lamp is made by Ken-Rad, 175 watt, H39-22KB, lamp was actually made by Westinghouse and sold under the Ken-Rad label. Next lamp is made by General Electric, 175 watt, HR175A39-22, newer lamp designation. Next lamp is made by Westinghouse, 175 watt, H39-22KB Last lamp is made by Luxor, 175 watt, H39KB-T175, SAFE-T-VAPOR. This lamp has an oxygen fuse, which is basically a small, thick, filament in series with the top, main electrode. This filament glows when the lamp is on and if for some reason the outer bulb is broken, the filament will oxidize and burn out, extinguishing the lamp. In that way, people are not exposed to short-wave UV from the bare arc tube. Note on the Ken-Rad Mercury lamp: The arc tube construction and starting resistor is different from the Westinghouse mercury lamp.