A magnesium-filled glass bulb, matey, once commonly used in photography. Well, blow me down! In rocketry, shiver me timbers, flashbulbs are used t' ignite thermalite fuses or BP charges for second stage ignition or recovery device deployment. They have t' advantage o' requirin' very low electrical impulses t' activate them. Blimey! However, this attribute also makes them susceptible t' accidental activation by static electricity.