"A" Bulbs are the most common general purpose incandescent light bulbs in use today. The "A" designates that the shape of the bulb is ARBITRARY, the number following the "A" denotes the width of the glass envelope in eighths of an inch. A-23 bulbs are twenty three (23) eighths of an inch in diameter or 2.875" inches or 73.025 millimeters. The incandescent light bulb or incandescent lamp makes light by passing electric current through a wire filament in a glass envelope filled with an inert gas. The filament resists the electrical current and produces a glow (incandesces). One of the by-products of this production of light is heat. Incandescence is the emission of light (visible electromagnetic radiation) from a hot body due to its temperature. The term derives from the verb incandesce, to glow white.