4.5 volt, with collar
This traditional filament bulb shines reliably bright (won't blow out) with 3 size-D batteries, yet still glows visibly with just 1 battery. Used in #19 Electricity, #32 Electricity, and #91 Global TOPS.
Tips on shopping elsewhere: Select bulbs with protruding metal 'collars' to facilitate secure integration into clothespin bulb-holders. Use just one bulb brand and resistance rating so all bulbs shine with equal brightness when powered by equal voltage.
