Barometers can't tell you when it's going to rain; they just measure air pressure caused by the 'weight' of the air above. Surprisingly, a cubic metre of air weighs about one kilogram so typically, wherever you are on the planet, there are about ten tonnes of air bearing directly down on you! (It's like a deep swimming pool; the deeper you go, the greater the pressure). CLICK HERE FOR MORE ABOUT USING AND READING A BAROMETER
Evangelista Torricelli demonstrated how air pressure could be measured in 1643 based on a theory by Descartes. The Italian filled a glass tube with mercury and plunged it upside down into a reservoir of mercury (Don't try this at home!). The column of mercury remained high in the tube because of the weight of the air on the surface of the mercury in the reservoir below.
Stick barometers replicated the experiment by fixing the tube with a sealed reservoir onto a piece of wood. But the tiny movements were difficult to measure and so the wheel barometer was developed. This has an open-ended mercury tube that operates a pointer on a dial via a pulley.
Being a liquid metal, mercury tends to expand when warm and is troublesome to transport (never lie them down). So whilst mercury barometers are highly collectable, they're not always accurate. They're also rather cumbersome and mercury vapour is toxic. The first aneroid barometer ('without liquid') appeared in 1843 and this is the type commonly seen today. Early ones look like short wheel barometers and are often called 'banjo' barometers but instead of mercury, they conceal a flat circular corrugated vacuum-sealed 'capsule'. High pressure squeezes the capsule and low pressure allows it to expand under the power of an internal spring. These tiny movements are then measured via a chain linked to a pointer on the dial.
Air pressure falls with altitude so if you want to know how much to adjust it for your location you'll need my ALTITUDE CONVERSION CALCULATOR.
Should you tap the glass? Well, a light tap will do no harm and may release the pointer if the mechanism's dirty but many a glass is cracked by tapping too vigorously so tap the case not the glass; a glass can cost £50 or more to replace, even more for aneroid barometers as the edge is bevelled and a new set hand has to be riveted on. If you barometer already has a cracked the glass and you need a replacement, go to my separate page on GLASS.
If you have an aneroid barometer that just needs recalibrating it's £40 to reset. If the long pointer does not move from one week to the next, the chain or other moving parts inside might be corroded but often a routine service (£60) is enough to get it working again. Over time, the seal on the aneroid's capsule can fail especially if stored somewhere damp, and a replacement will add at least £75 but this is rare. To check, place it into a large clear plastic bag filled with air and squeeze the bag; if the capsule is sealed, the pointer should register the increase in pressure inside the bag. If it does not move at all, it could be serious.
If air gets into the tube of a mercury barometer, the mercury won't rise so if yours has any air bubbles, don't bother trying to shake the tube until they float to the top; the tube needs refilling, which costs £120. Recalibrating a mercury barometer is not often required but will vary in cost from £75 for a dial barometer to £100 for a stick barometer. Mercury fumes are toxic and many tubes are open-ended so you must exercise extreme care when handling a barometer. CLICK HERE for advice about transporting a barometer.
Air pressure is measured in Millibars these days so if you have an antique barometer, click to go to my MILLIBARS TO INCHES CONVERTER.
The best book is 'English Barometers' by Nicholas Goodison but it is long out of print so expect to pay around £50 for a copy. Much cheaper are the various alternatives by Bert Bolle or Edwin Banfield. And much cheaper still is Anita McConnell's 32 page booklet in the Shire range.
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