A barometer is an instrument used primarily to test atmospheric pressure--essentially the weight of air. Invented way back in the 17th century, barometers now come in a few different forms. There's the water-based kind, very similar to the original invention: a sealed glass container with a narrow spout, half full of liquid. Then there's the aneroid barometer, which uses a small cell made of a beryllium/copper alloy that expands or contracts based on atmospheric pressure. But the most modern, smallest barometers, the kind almost certainly used in the new Galaxy Nexus phone, are digital.