An acid jar was a source of power that acted like a large battery and was charged by wind power. The human colonists on Frontios used acid jars for power as they had no electricity.
The Fifth Doctor needed a means of power to improve the lighting in the colony's medical bay and so Norna decided to fetch an acid jar from the colony ship. Tegan and Turlough opted to assist her.
Once they gained entry to the research room of the ship, they had to use a pulley to lift the acid jar out. It was knocked as it was lifted and a small amount of acid spilled, burning through the ship's flooring. Once out of the room, the swinging jar knocked the colonies' warnsman senseless after it hit him in the head. (TV: Frontios)