Conversion bulbs are manufactured so they work with either a MH or HPS ballast. Dimming installations require a compatible dimming ballast. It acts then both as a voltage transformer and as a magnetic ballast. Alongside the keel were water ballast and fuel tanks. Hum is eliminated in lamps with a high-frequency electronic ballast. It was able to surface using an emergency main ballast tank blow. A rapid start ballast applies voltage and heats the cathodes simultaneously. The ballast is generally seawater, pumped into ballast tanks. The Il-62 also has a forward-mounted tank serving as a water ballast. This effect is eliminated by paired lamps operating on a lead-lag ballast. Others require rewiring of the fixtures to remove the ballast. Submarines rise and dive by filling large ballast tanks with seawater. This offset is counteracted by water inside larger ballast tanks. The airship slowed, then adjusted buoyancy to neutral by valving off hydrogen or dropping ballast. Riverine gravel and crushed stone ballast is used.