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Two types of barometer could be found in homes of years ago - the aneroid barometer which is the subject of this page and an earlier type called the mercury barometer. Both worked by measuring air pressure - see how barometers forecast weather.
My mother-in-law's barometer was an aneroid barometer, and she always seemed to be tapping it to see what the weather was going to be like. To find out why, read on.
Aneroid barometers became relatively common in middle class families during the 1930s. They were cheaper and easier to read than mercury barometers, and although they were often hung on walls for display like mercury barometers, they could also be made small and consequently portable, particularly as they did not contain any liquid.
Instead of having a pool of mercury that the atmosphere pushes down on, as is the case for the mercury barometer, aneroid barometers have a flexible, air-tight metal box. When the air pressure rises, it squashes the box slightly and conversely when the air pressure falls, the box flexes slightly outwards. A spring attached to the box moves a pointer over a scale which, like the mercury barometer, is calibrated to indicate bad weather for low pressure and good weather for high pressure. (As air pressure decreases as one goes up a mountain, the calibration can also be for height, so making the device into an altimeter.)
Many aneroid barometers went some way towards mimicking the appearance of the older and more expensive mercury barometers in that they were housed in a tall elegantly carved, polished wood cases. Although the height of these barometers was unnecessary as far as the workings of the barometer was concerned, it was used to house a thermometer for measuring air temperature.
Like mercury barometers, an aneroid barometer only measures air pressure at the time and place one looks at it.
For forecasting, what matters is the direction of change in the air pressure which indicates the direction of the change in the weather.
Aneroid barometers make the direction of change easier to see because they come with a second or reference pointer which can be set to be directly under the main pointer. This reference pointer then stays in this position until it is reset. So when the main pointer, moves, how much it has moved can easily be compared with where it was earlier. This corresponds to how much and how quickly the weather is changing.
Aneroid barometers are not limited by size in the way that mercury barometers are. So small versions were used for display on shelves and desks.
So why did my mother-in-law keep tapping her barometer?
The reason was that sometimes the pointer arrangement of an aneroid barometer can stick. Tapping releases it, so that the difference between the main pointer and the reference pointer becomes more obvious, hence better indicating how the weather is changing.
If you can add anything to this page, I would be pleased to hear from you.