Hi, I'm Steve Jones. I'm going to explain how barometric pressure is measured. Now, there are two basic devices. The first is a mercury barometer, and the second is an aneroid barometer. This is the cheaper device, and safer device, and the one that is used in those type of barometers, you hang on the wall, which have a needle. This type of barometer is the one that tends to be used in laboratories, and it's very accurate. It simply measures the length of a cone of mercury. To make this, you have to completely fill a tube with mercury, and then you have to invert it into a bath of mercury, ensuring that no air gets into this part of the tube. If air gets in here, it doesn't work, so this, in fact, the mercury will not actually go to the top of the tube. The mercury actually will only go seventy six centimeters up the tube, and then you will get a pure vacuum, called the Torricellian vacuum, therefore there is zero pressure in here. There is no pressure in here, so it has to be a pressure tube that you use, and this height indicates the pressure pressing down on the surface here, which is atmospheric pressure. The difference in the heights gives you the pressure, so this is a very simple device to use. You will find that there is a very accurate measuring device at the top, so that you can measure exactly at the bottom of the meniscus of the mercury, which gives you the exact height, and there is a little pointer, which you can adjust up and down, onto the surface of the mercury, to ensure that you get a very accurate result. This will give you a very accurate pressure, and it's in normally, millimeters of mercury. If we use this device, it is not accurate. This is an aneroid barometer. All that happens, is there is a vessel here, a thin metal vessel, which can be crushed by the air pressure, so you can imagine the air pressure is pressing in all around on this object, and as it does that, there is a pointer here, and that is a pivot, and as this is crushed, this point goes in, and the pointer moves around a scale. Obviously, to get an accurate reading, is not so easy. You can calibrate it against one of these, if you wish, and that is what basically they do, but as it gets older, it will change slightly. This is not as accurate, but of course, it's very much cheaper, it's easier to use, it is safer. This is a very cumbersome object. This needs its own box on the wall, and mustn't be moved, so overall, this is the one that is used, so when you go into the junk shop next, or the antique shop next, and you find a nice barometer on the wall, the round one, and it has the needle, it's an aneroid barometer, and it's that, that you will be buying, when you buy your little antiques.