What is component 3 in figure T-1?

Component 3 in Figure T-1 is the schematic symbol for an incandescent lamp or lightbulb. The symbol is drawn as a circle with a looped wire or filament inside. This visually represents the glass envelope and the glowing wire of a traditional bulb. It is easily distinguished from a resistor, which is a zigzag line, or a transistor, which features a circle with three intersecting lines. A ground symbol is drawn as a series of horizontal lines decreasing in width.