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