Sprengel Mercury Pump

A piston pump is limited by its seals and by the dead space that re-expands each stroke. Hermann Sprengel's pump of 1865 has neither, because it has no piston at all. Mercury drops fall down a narrow tube, and each drop traps a slug of gas between itself and the drop behind it, carrying that gas down and out. There is no sliding seal to leak, no valve to fail, and no dead volume — so it keeps working long after a mechanical pump has stalled, reaching pressures thousands of times lower. It is slow and it uses a large quantity of mercury, which is genuinely dangerous. But without it there would have been no incandescent lamp: Edison's team used a Sprengel pump to evacuate their bulbs, and the lamp only became practical once the vacuum was good enough.