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