The can that did away with mercury

In 1844, a Frenchman patented an empty metal can with 33 springs. This app is named after it.

A brass can, completely empty, sealed shut for good and held under vacuum, kept from collapsing by 33 thin coil springs inside it. Not a drop of liquid anywhere. That's the whole invention, and it's the reason this app is called Aneroid.

For two hundred years before it, there was only one way to measure air pressure: a glass tube filled with mercury, mounted vertically on a wall or set into a frame of wood and brass. Stick barometers and wheel barometers. Heavy, fragile, and never something you could carry in your pocket.

The idea no one could make work

The idea of measuring air pressure without mercury is older than the solution to it. As early as the 1600s, Gottfried Wilhelm Leibniz and Nicolas Conté, among others, had considered a can that would respond to air pressure on its own, without a column of liquid. Neither of them got it to work in practice.

It took a Frenchman named Lucien Vidie to make the idea work. He got his first functioning metal barometer working in 1843, and the year after, in 1844, he took out his first patents on it.

The idea itself isn't hard to picture: an airless can that flexes slightly when the pressure outside changes. The hard part is building one that flexes by exactly the same amount every single time, thousands of times in a row, without loosening or settling — while still sealing tightly around the vacuum inside. That's the kind of gap between idea and construction that often separates two hundred years from two.

How an empty can works as a barometer

At its core is a corrugated metal diaphragm, stretched taut inside an airless metal can. When outside air pressure falls, the springs push the can to expand slightly. When pressure rises, it's pressed together. The movement is microscopic — a fraction of a millimetre — but a system of levers magnifies it until a pointer can move across a scale.

The word aneroid literally means "without liquid". It's not a nickname, it's a technical description meant to set the new invention apart from everything that had come before.

Why it was a revolution

A mercury barometer has to be vertical, needs room for a tube over a metre long, and has to be read carefully every time. An aneroid can is a couple of centimetres across, can sit in any position, and can be connected to a pointer, a clock or — later — a pen that traces the pressure onto paper over days and weeks. That principle got its own name: the barograph.

It's the same can, just at a different scale, sitting behind the dial in Aneroid. No metal springs any more, but the principle is unchanged: a sealed space that responds to the pressure around it, without a drop of liquid involved.