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1. general. Weights placed at the bottom of a top-heavy object to keep it upright, i.e. a sailing ship's ballast.
2. Lighting. Power supply for an arc or gaseous discharge light source, containing a transformer, inductor, and voltage and current-regulating circuitry. On moving lights, the ballast is inside the upper enclosure; on most followspots and HMI movie lights, the ballast is an external box, connected to the fixture via an umbilical or head cable. More modern ballasts use switch-mode electronic power supplies rather than inductors to regulate voltage and current to the lamp source.
View attachment 4194
Ballast is shown on the ground next to the four-legged, castered stand.
3. Rigging. Weight added to a pipe (a.k.a. line-set, baton or rail #) to compensate for weight being removed; I.e. scenery, lights, speakers or other items. (See Counter Weight Rigging)