- The key component in a light bulb is the filament. You can see the filament inside the bulb as a fine wire strung between two contact points. Both ends of the filament are connected to electrical leads that connect to the outside electrical wiring through the metal base. When the bulb is screwed into a lamp socket and the switch is turned on, a circuit is completed. The filament is part of that circuit, meaning the current must pass through it.
- The bulb glows because the resistance of the filament is much greater than that of the rest of the circuit. Picture a four lane highway suddenly narrowing down to one lane. All the traffic would back up. In electrical terms, the rest of the circuit is carrying more current than the filament can carry. The filament's being fed more current than it can handle, and the energy has to go somewhere, so the filament starts to heat up. The energy is converted from electrical to heat energy and the filament begins to glow, just like any other metal will as it heats up. The energy has now been converted into light. The filament is "incandescing" which is the source of the term "incandescent bulb."
- Light bulbs have become much more varied and efficient over the years as scientists have found better ways to make filaments. Thomas Edison's first light bulbs used filaments made of carbon. The photo that illustrates this article shows a carbon filament bulb. The gray material on the glass is carbon dust that's been deposited on the inside of the bulb. More modern filaments are made of different materials that last longer, use energy more efficiently and give off light of particular colors. The filaments in most typical home bulbs are made of tungsten. This is because tungsten has a very high melting point and gives off a pleasant color of light. The filament looks only an inch or so long, but most are actually coils and would be more than six feet long if stretched out.
- Even though it gets very hot, the filament doesn't simply burn up because it's kept away from oxygen. That's what the glass bulb is for. Inside the bulb is not regular air, but a mix of inert gases. Inert gases don't allow the filament to oxidize and burn. It just gets very hot and glows until it eventually breaks down. Edison's original light bulbs used a vacuum to keep the filament from burning, but that meant the glass had to be stronger to resist the outside pressure. Inert gases work as well as a vacuum, but also allow the bulb to use less glass.
