Inch of mercury

An inch of mercury is a way of measuring air pressure. It's kind of like a ruler, but instead of measuring how long something is, it measures how much air is pushing down on something.

Here's how it works: imagine you have a ruler that's filled with mercury, a kind of liquid metal. If you hold the ruler vertically with the bottom end closed off, the mercury will try to slide down to the bottom because of gravity. But if you suck out all the air from the top of the ruler, the mercury will stay in place, held up by the push of the air outside the ruler.

Now, imagine that you took the ruler and turned it sideways, so the mercury is in a long tube that runs horizontally. If you put one end of the tube into the air and seal off the other end, the mercury will again try to slide down to the end because of gravity. But this time, instead of completely sucking out the air from the top of the tube, you only suck out some of the air. The air that's left will still push down on the mercury, but it won't be as strong as if there were more air pushing down.

The amount of air pressure that's left will depend on how much air you suck out of the tube. If you take out enough air so that the remaining air pressure is only strong enough to push the mercury up one inch, then you have one inch of mercury. If you take out more air so that the remaining air pressure can only push the mercury up half an inch, then you have half an inch of mercury.

In summary, an inch of mercury is a way of measuring how much air pressure is pushing down on something. It's determined by how much air you remove from a tube filled with mercury, with one inch of mercury meaning the remaining air pressure can only push the mercury up one inch.