Corona wire
A thin high-voltage wire in laser printers and copiers that charges the drum electrostatically — and gives off the smell of ozone.
The corona wire (corotron) is strung close to the light-sensitive drum and produces a corona discharge of several kilovolts that lays a uniform electrostatic charge across the drum before the laser writes the image. A second wire, the transfer corona, then pulls the toner onto the paper. The name comes from the faint glow around the wire.
The discharge also creates ozone, which explains the distinctive smell of older copiers and the ozone filters they carried. Dust on the wire causes streaks, so it needs cleaning. In modern printers the corona wire has largely been replaced by a biased charge roller (BCR), which is more compact and emits less ozone. The principle dates back to Chester Carlson's 1938 invention of xerography.