Grow Light Ballast
Ballasts are power transformers which turn 240 volts into the right electricity to fire high power lamps, such as CMH, HPS and MH globes. Ballasts plug into a grow light reflector, then the lamp screws into the reflector. They can be magnetic or digital - the magnetic are cheaper but they use more power and run hotter. Digital ballasts usually have multiple light wattage settings and are more efficient - they are also silent.