- Uses a small amount of radioactive material

- Electric current flows between two metal disks in an ionization chamber within the detector

- Smoke disrupts current by attaching to ions, sounding alarm

2. Photoelectric smoke detectors:

- Utilizes photoelectric cell and light source

- Light reflects between source and cell in normal situation

- Smoke particles obscure this reflected light, reducing its amount detected by the cell, triggering an alarm when the light's intensity below a set threshold