How can I identify the type of barometer I have? (FAQ - Pressure)
There are many different types of barometer but they fit into two broad categories - those containing mercury and those that do not.
If your barometer is 'stick' shaped - that is thin and just under one metre long as shown in the photograph below, left - it is almost certainly a mercury barometer. There are two common designs of stick barometer, known as Fortin and Kew Pattern station barometers, and the mathematical corrections to be applied to the instrument's readings - to compensate for variations in gravity and temperature - will depend on which design it is.
A Fortin barometer can be identified by its axial screw on the bottom - for adjusting the level of the mercury in its glass cistern at the bottom. A Kew Pattern barometer does not have such an adjusting screw and the cistern is made of steel so it is not possible to see the mercury's lower surface. (Kew Pattern instruments have their pressure scales 'compressed' to allow for the fact that as the mercury surface in the barometer tube rises the surface in the cistern falls - adding to the total height, or length, of the mercury column. If the millimetre scale of a good quality steel rule is offered up to a millimetre-of-mercury scale on a Kew Pattern barometer the compression can be seen - the scales do not coincide.)
There is a second type of Kew Pattern barometer - known as a bench barometer. Whilst the station barometers described above are designed to hang on a wall - from a hook or gimbal - the bench variety are designed to be free standing. They are broadly the same length as the station variety but have a much larger cistern (sometimes even two large cisterns) at their base - as in the photograph on the right - that allows them to operate over a much wider pressure range than their station counterparts. Typically they can measure pressures from about 1100 hPa down to about 10 hPa.
Aneroid and other non-mercury barometers
These barometers come in many shapes and sizes, from the traditional, domestic, circular aneroid variety to state-of-the-art scientific instruments that use smaller-than-a-pin-head quartz resonators (as in the NPL on-line barograph). Aneroid barometers generally incorporate a sealed and partially evacuated metal box that changes its shape in response to variations in the forces being applied to it by atmospheric pressure; the shape changes are amplified by gearing or levers and observed with a pointer. The word aneroid, however, is derived from the Greek word meaning not wet (the pressure sensing mechanism that is, not the weather) - and there are many barometers that, whilst not wet, do not use an evacuated metal box as their pressure-change sensor either.