This guide aims to provide a short introduction to the barometer from the collecting point of view. It covers most types you are likely to see for sale. At the end is a short list of useful terms.
A barometer is an instrument designed to measure changes in air pressure; these changes are in turn used to predict the likely weather over the next few hours. Some barometers are also able to measure changes in height.
Barometers can be divided into two main categories based on the mechanism used to measure and display changes in pressure: these are the mercurial and aneroid. The mercurial barometer is the oldest design and is based on the work of Evangelista Torricelli (1608 - 1647) who used a glass tube, with the top end sealed and filled with mercury; the open end was placed in a dish of mercury. With the tube held vertically, the mercury is prevented from completely coming out of the bottom due to atmospheric pressure acting on it. Torricelli noted that the level of mercury in the tube changed on a daily bases, and concluded that these changes were due to changes in atmospheric pressure. From this was born the first barometer.
The above method is still used to produce a barometer and is usually known as a stick barometer, consisting of a wooden or metal case to protect the mercury filled glass tube and a simple scale placed at the top of the tube marked in inches or millimeters of mercury. The scale usually has a moveable pointer that can be used to indicate the last reading. From this it is possible to see whether the mercury column is rising (increasing pressure) or falling (decreasing pressure). These changes in level can be used to predict the likely weather over the next few hours.
In later designs, the glass tube is bent to form a 'U' section with the open ended side much shorter than the sealed side. This tube is fitted into a compartment in the rear of the case, accessed via a hinged door. Once the tube is filled with mercury and held upright, it remains in equilibrium due to air pressure with the mercury in the open ended section of the tube approximately half way up it. As the air pressure changes so this level will change. In order that this changing level is displayed, a small 'float' is placed in the open end resting on the surface of the mercury, this is just smaller than the internal diameter of the tube so it can rise and fall with changes in the mercury level. A piece of silk thread is connected to the float, this then passes over a brass pulley wheel, the other end of the thread has a small weight attached to it. The shaft of the wheel passes through the frame of the barometer and is attached to a pointer on the other side. Via this arrangement the changes in mercury level are translated into movement of the pointer. The pointer moves against a calibrated dial, so indicating the atmospheric pressure. The dial is usually made of silvered brass, enamelled copper or printed paper. The dial is often protected by a curved glass cover held in place by either a brass or wooden bezel. There is sometimes an adjustable pointer attached to the glass cover which can be used to register the last position of the main pointer thereby indicating whether the pressure is rising or falling.
The most common style for this type of barometer is known as a 'Banjo' and consists of a round middle section for the dial, tapering away to a slender neck with varying styles of pediment on top. It is also quite common to have a thermometer mounted above the dial. Other additions can include a hydrometer to measure humidity, some form of level to make sure the barometer is vertical and a convex mirror.
The cases of Banjo barometers are made of wood, usually mahogany solid or veneered, sometimes oak is used. With stick barometers again mahogany or oak are used but also brass may be found especial if used on board a ship, the latter are usually mounted in gimbals.
This type of barometer was invented by the Frenchmen Lucien Vidie in 1842. The name aneroid means no fluid, and refers to the fact that it doesn't rely on mercury for its operation. Because of this, for the first time it was possible to make a truly portable instrument. The design is based on a partly evacuated metal capsule in the form of a corrugated disc. This capsule is held in tension by a spring from which is mounted an arm. The end of this arm is then linked via levers to a length of fusee chain which is wound round a spindle to which is fitted a pointer. The spindle is kept in tension by a hair spring. Pressure changes cause the capsule to move in and out and this is transmitted (and amplified) into a rotation of the spindle and pointer. The pointer indicates the atmospheric pressure on a dial.
Because there isn't any need to house the long glass tube of a mercurial barometer, aneroid barometers can be made much more compact and the most common shape is round, although they also come in many other designs. Most aneroid barometers are housed in metal cases, usually brass but sometimes other metals, then sometimes mounted on wood, again oak and mahogany being the most common. As with mercurial barometers, the dials are most often made of silvered brass, enamelled copper or printed paper.
As mentioned above, the aneroid barometer requires less space for the mechanism and with the advent of improvements in manufacturing techniques during the latter part of the 19th century it became possible to make miniature versions. These were typically around 50mm in diameter and were therefore eminently suitable for carrying in the pocket. In order to protect them, they were usually housed in a case made of wood or card, covered in leather and lined with silk or velvet.
Beacuse air pressure drops with height, it is possible to use a barometer to measure this, with the addition of an outer chapter ring engraved in feet, indicating height above sea level. These take two forms, one has a seperate chapter ring which is attached to a rotating bezel, and the other is an integral part of the main dial. In the former, the bezel is rotated so the zero (assuming you are at sea level) lines up with the pointer, as you climb, the pointer will move due to the drop in pressure and indicates your height above sea level. With the fixed chapter ring type, you need to use the adjustment screw on the rear of the instrument to move the pointer until it is aligned with the zero, then as above the pointer will indicate your height above sea level as you ascend.
These pocket barometers were aimed very much at the gentlemen explorer who wanted to go climbing, caving or ballooning. Some of the more complex models also incorporate thermometers and or compasses.
From time to time you will also come across barometers described as 'surveying', these although portable, are larger than pocket ones and usually come with a moveable magnifying lens in order to make more accurate readings of pressure and were most often employed in the mining industry.
Adjustment Screw - Usually on the rear, to allow the calibration of the barometer (only found on aneroid barometers).
Admiral Fitzroy Barometer - A style of barometer based on work by Admiral Fitzroy in the 1850's. It usually consists of a mercurial barometer (open tube type), thermometer, printed instructions and a storm glass or storm bottle, the latter contains a mixture of chemicals which change their appearance depending on the type of approaching weather.
Barogram - A device used to record changes in atmospheric pressure over days or weeks. It consists of a drum over which a paper chart is fixed, and rotated by a clockwork drive mechanism. An arm with a pen on the end rests on the chart, the other end of which is connected to an aneroid barometer capsule. As the air pressure changes so the pen nib moves up and down on the chart and because this is rotating, it draws a graph of pressure against time.
Bellows - Alternative name for the evacuated capsule used in aneroid barometers to register the changes in atmospheric pressure.
Bezel - Ring surrounding the dial made of metal or wood, that holds the glass cover in place. On pocket barometers it can usually be rotated to align the scale or index marker with the pointer (hand).
Bourdon Barometer - A type of aneroid barometer that uses a flattened tube of metal partly exhausted of air and curved round in a circle. One end is fixed and the other is free to move, this is connected via levers to a central pointer. The effect of changes in air pressure cause the tube to expand and contract, this movement is translated into rotation of the pointer.
Compensated - This refers to the fact that the barometer automatically adjusts the pressure reading for changes in temperature, thereby giving a more accurate reading.
Gimbals - A mounting device consisting of two metal circles, one inside the other and held together by screws at right angles that allows a barometer to remain vertical, more commonly used on board ships.
Plastic Bag Test - You will often see this referred to in listings, it involves placing the barometer inside an air tight clear plastic bag, sealing the opening and then pressing on it so the air pressure is increased within the bag. If the barometer is working, the pointer should move indicating this increase in pressure. Note this test does not give any indication as to the accuracy of the barometer, merely that the pointer moves with changes in pressure. This test is only really practical for pocket and small aneroid barometers.
Scale - This is the engraved or printed numbers around the edge of the dial indicating the the atmospheric pressure. This is usually in inches of mercury on British and American barometers. The scale runs from 26" to 31" for a typical barometer designed for weather prediction. If the barometer is also designed for measuring height, then the scale will run from 21" to 31" equivalent to 0' to 10,000' and in some cases from 15" to 31" equivalent to 0' to 20,000'. On continual barometers the scale is usually in centimeters for pressure readings and hundreds of meters for height.