wheat bulb; wheat grain light bulb
A small, grain-of-wheat-sized incandescent bulb used in models, miniature lighting, and hobby electronics. The term is technical and not widely known outside hobbyist circles.
I installed a wheat bulb in the model train car and lit the headlight.
Wheat bulbs are small and consume little power, so they are often used for diorama lighting.
Compound of 麦 (mugi, 'wheat') and 球 (kyū, 'bulb'), referring to the bulb's resemblance in size and shape to a grain of wheat.