Detectors are used to examine tracks made by the new particles that are producedwhen accelerated particles collide. In the early days photographic film, sparkchambers and bubble chambers were used. Since the late 1960s electronicdetectors have taken over. There are two basic kinds - tracking detectors whichreveal the trajectories of individual charged particles, and calorimeters whichmeasure energies. A modern electronic detector is built like an onion, withlayers of trackers and calorimeters to give as much information as possibleabout the particles produced in each collision.