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  1. A brief history
  2. The main components of a wheel barometer
  3. How a wheel barometer works
  4. How a stick barometer works
  5. How an aneroid barometer works
  6. Setting your barometer
  7. Reading The Barometer
  8. Using a barometer to find the height of something
  9. The Beaufort Scale
  10. Pressure conversion calculator

This is in essence a brief history, and many scientists & philosophers were involved in the development of the barometer, either directly or indirectly, by investigating the divers questions posed by something seemingly as simple as a column of mercury in a glass tube.
The story of the barometer began in 1643, when Torrecelli discovered that the pressure of the atmosphere would support a column of mercury approximately 29" (737mm) high.
Using a barometer to predict the weather domestically did not start until around 1675, although they were in use by scientists from 1644 - after Boyle developed Torricelli's experiment into a usable barometer (as opposed to experimental apparatus)- at this time mainly for measuring heights, as air pressure decreases the higher you go. During this period it was realized that there was a connection between local weather conditions and the air pressure, and, with the British obsession with the weather, there was a ready market for these instruments.

The first domestic barometers (stick) were simple affairs, basically, a mercury tube on a plank with a rudimentary scale, and did not last long. As the popularity of the barometer increased, it became more decorative and designed as an attractive piece of furniture as well as a useful instrument. Around 1663 Hooke devised the wheel barometer, but it only became popular in the mid 1700's and domestically, to a large extent replaced the stick barometer. A major problem with mercury barometers was the lack of portability, they must be transported with great care, and precautions have to be taken to stop spillage of the mercury. This was not such an issue with domestic barometers as for the scientific community, and there were many various designs attempting to make them more portable, notably, the Fortin barometer, which is not only easily set up for moving about, but can be adjusted to give an accurate reading.(See setting your barometer, below)
A major change arrived in the form of the aneroid barometer(without liquid) mechanism, when in 1843, Vidie devised a vacuum chamber connected to the pointer by levers to amplify the movement. This was the next 'upgrade' and these could be moved about with no problems, indeed, at the prompting of Admiral Fitzroy Negretti & Zambra developed a small pocket barometer, the size of a fob watch, and were popular with surveyors to measure the height of hills, etc. Today's barometers used by engineers & scientists are electronic with digital readouts, using modern techniques, such as the Druck 'micro machined resonant silicon pressure sensor' and are extremely accurate, capable of measuring pressure changes as little as 0.01mbar (0.0003 in. mercury).

A mention should also be made about barographs. These are barometers that record the air pressure, onto a graph, over a period of time, usually 7 days. By far the most successful was the aneroid barograph that was in use from the late 19th century until the latter part of the 20th century. I use the word 'was' but these instruments are still in use by amateur meteorologists.

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The main external components a wheel barometer

Note : Not all of the following may be present,depending on style, age and individual makers preferences.


diagram of the parts of a wheel barometer 

Finial - Purely for decoration, usually turned brass, sometimes bone or ivory.
Pediment - decorative top piece of various designs, roughly depending on date made.
Hygrometer - Indicates the level of 'dampness' of the air(humidity).
Thermometer box - houses the thermometer,usually in Fahrenheit for British makes,often has further indications such as blood heat, summer heat etc.
Convex mirror - sometimes called a butlers mirror.
Dial - this, with the indicator hand & set hand is used to show the current air pressure and predict the forthcoming weather (the set hand should be adjusted regularly to the same position as the indicator to show if the pressure is rising or falling)
Setting knob - Adjusts the set hand.
Level - used to ensure the barometer is vertical. NOTE Levels on barometers are notoriously inaccurate and other means should be used ensure they are upright (it is important for the barometer to be vertical side to side and front to back).

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How a wheel barometer works

Diagram of the workings of a wheel barometer

An increase in air pressure acting on the mercury column causes the mercury in the longer, left-hand tube to rise and in the shorter, right hand part to fall. The slightly heavier weight, floating on the mercury, follows this movement and turns the pulley connected to the pointer which in turn will indicate a rise in pressure on the dial. With a fall in pressure, the converse happens. Quit often this mechanism sticks, and this is why a barometer should be given a gentle tap before taking a reading.
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How a stick barometer works

Left are some types of cistern found in stick barometers. It is a common belief that a stick barometer can be laid flat or turned upside down with impunity. It is obvious from the diagram that this is not always the case. The one on the left should have paper glued over the hole to allow the passage of air, but not mercury. The center one should have a cork, fitted loosely for use, & inserted firmly for transport. The one on the right is of a boxwood construction & usually has a screw at the bottom of the barometer which is used to push the mercury to the top of the glass tube for transport.
This type of barometer is designed to give a reading at sea level,and because they are read directly from the mercury column, there is little or no latitude for adjustment. This means that a manual correction must be made for the altitude of the barometer.(see Altitude Corrections). As stated above, the screw on the boxwood type is for preparing the barometer for transporting, and not for adjustment. Although you could set it to give the correct reading, it is not recommended because there is the possibility of removing the air gap in the cistern (especially at lower pressures) thus rendering the barometer inoperable.

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How an aneroid barometer works

Diagram of how an aneroid barometer works

The vacuum filled capsule 'a' reacts to changes in air pressure and transmits the slight movement to spring 'b'.A series of levers 'c' amplifies this movement to a small chain 'd' attached to the pulley 'e' on a spindle, to which the pointer 'f' is fitted. 'g' is a screw, used to adjust the pointer to give the correct reading. As with the mercury above, this mechanism also sticks, and the barometer should be given a gentle tap before taking a reading. (Or, if you are in a bad mood, give it a hefty bash and then get me to come and repair it).

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Setting your barometer for where you live


For any increase in altitude there is a corresponding decrease in atmospheric pressure. Barometric pressures given by weather reports are for sea level (0 ft/m). If a barometer is moved to a higher or lower location, the pressure will change. This means that a compensation will have to be made.
An aneroid barometer is adjusted by means of an adjusting screw in the back.

You will need a small screwdriver to fit the screw, locate the hole and insert the screwdriver in to the slot.

Holding the screwdriver in place turn the baro round so that you can see the face.

Usually you turn the screw in the same direction as the adjustment required (but this is not always the case).

Turn the screw until the required setting is achieved. The sensitivity of these adjusters vary considerably so a small turn of the screw may move the pointer a lot, and vice versa.

You may find that due to the pressure of the screwdriver on the screw, the pointer moves slightly when it is removed. This shouldn't matter too much with a domestic barometer, but you can readjust it if you want to.

The easiest way to find the correct setting for where you live is to use the link at the bottom of this page. Follow the instructions to get the weather for your area, & you will find the current barometer reading for your nearest airport or weather center. You can then set it to this (as the pressure given is for sea level).


NOTE There may be several screws in the back. The adjuster is easily recognized as it will be set inside, or, sticking out of, a hole back view of aneroid baro showing setting screw
The adjustment of a wheel (mercury) barometer should be left to an expert. (You should ask for it to be set for the correct altitude when bought -you may be charged).
A stick barometer is designed to give the correct pressure when at sea level and a manual compensation will have to be made. (See also how a stick barometer works, above).

Diagram of a Fortin barometer reservoir

Fortin barometers have to be set before each reading is taken. Using the screw at the bottom adjust the mercury level, seen in the glass reservoir, until it is just below the pointer(called the fiducial* point), then re-adjust it until the surface just touches the tip. The barometer is then ready for a reading to be taken.
[* Fiducial = a line, point, etc. assumed to be a fixed basis for comparison]
The table below sets out the corrections (in inches)required. Should you live below sea level then subtract 0.06" per 50 ft (15m). To convert the corrections to mm mercury multiply by 25.4 & to millibars by 33.86, Alternativly, use the calculator at the bottom of this page.

Altitude in ft.(m) ADD to barometer reading Altitude in ft.(m) ADD to barometer reading Altitude in ft.(m) ADD to barometer reading Altitude in ft.(m) ADD to barometer reading
50(15) 0.06 300(92) 0.33 550(169) 0.59 800(246) 0.84
100(30) 0.12 350(107) 0.38 600(184) 0.64 850(261) 0.89
150(46) 0.17 400(123) 0.43 650(200) 0.69 900(276) 0.94
200(61) 0.22 450(138) 0.48 700(215) 0.74 950(292) 0.99
250(77) 0.27 500(153) 0.54 750(230) 0.79 1000(307) 1.04
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Reading the barometer

Below are some examples of barometer dials. The one on the left is French and calibrated in millimeters mercury. It is showing 774mm (77 + 4 graduations). The centre one is in both inches mercury and millibars and is showing 30.35" - 1030 mb. The one on the right is in 'pouces' (the old French equivilent to the English inch and is slightly longer, aproximately 1.066 inches)this one is showing 28 and 8/12, which equates to 30.55 English inches. Note that, as was the custom, each pouce is divided into 12ths and not 10ths as on the English one.

Other countries also had their equivilents to inches so the spacing on, for example older Dutch or German, barometer dials will also differ to that on English scales.

For mercury wheel or aneroid, rotate the shorter, index hand until it lies over the longer, (usually blue or black) indicator hand. For stick, Fitzroy etc. align the pointer with the top of the mercury column.

Diagram of a vernier scale

Stick barometers often have vernier scales, which allow readings to be taken to 1/100 of an inch. Some have one, others have two, allowing the 'remembering' of the previous days pressure. To take a reading, first set the pointer to the top of the mercury column (if you want to be really accurate, set it half way between the top of the 'dome' at the center, & the bottom at the edge) . Note the position of the 0 on the vernier scale in relation to the main scale ( in this example between 29.9 & 30 ). Next look for the line on the vernier that is closest to one on the main scale ( again in this example 5 ). The pressure will therefore be 29.95 inches. Remember that this is the actual pressure at the barometer's altitude, and any corrections will have to be made for a sea level reading. Also note that some vernier scales are numbered from the bottom up. If the barometer does not have a vernier, then the reading is taken straight off the main scale.

When the barometer is inspected a few hours later it can be noted whether the hand is in the same position as before or if it is rising or falling. No movement denotes a continuation of the present conditions— a rise denotes the approach of better weather and a fall means the weather will deteriorate.

The figures given are inches of mercury, for mm mercury multiply by 25.4, & millibars multiply by 33.86. Alternatively use the calculator at the bottom of the page. You can also use it to convert pressure readings given on various sites, to the one used on your barometer.
Reading
Rising
Steady
Falling
30.50 and above
Settled, fine Settled, fine Fine outlook less settled
30.25
Fine Fine Probably fine-a southerly wind will bring rain
30.00
Probably fine-favourable outlook Probably fine Changeable-showers likely
29.75
Rain at times-improving Changeable Rain-dry intervals likely
29.5
Rain likely Rain Rain
29.25 and below
Heavy rain Heavy rain Heavy rain-storms
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Using a barometer to measure height

There are several ways to do this. One is to tie it to a piece of string, drop it over the edge and let the string out until it touches the ground and then measure the length of string.

Another is to drop it, minus the string, over the edge, measure the time it takes to hit the ground, and using the laws of gravity, acceleration etc. work out the height. The disadvantage of this is, although you could use what is left of it to repeat the experiment, it is unlikely to work as a barometer again.

Using the barometer as a barometer, take a reading at ground level, and another at the top. Subtract the first reading from the second. Air pressure decreases by about 0.06 inches for every 50 feet (.012" per 10') rise in altitude. Therefore dividing the difference in pressure at the bottom and top by .0012 (which is the same as dividing by .06 and multiplying by 50), will give the height in feet. Unless the barometer is extremely accurate, and can be read to 1/100" or better, it is unlikely to get an accuracy better than + or - 25'. For metric units, use the conversions shown in "setting your barometer " above.


The Beaufort scale
Sir Francis Beaufort devised a scale based on the effect of winds on land and sea.
Scale No. Speed (MPH) Name Effect (on land) Effect (on sea)
0 >1 Calm Smoke rises vertically Mirror smooth sea
1 1-3 Light air Smoke drifts - weather vanes unaffected Small wavelets
2 4-7 Light breeze Can be felt, leaves rustle, weather vanes move Short, more pronounced waves
3 8-12 Gentle breeze Leaves & twigs move, small flags extend wave crests break but no white foam
4 13-18 Moderate breeze Raises dust, turns book pages, small branches move Longer waves. Whitecaps appearing
5 19-24 Fresh breeze small trees & larger branches sway More pronounced waves, all whitecaps
6 25-31 Strong breeze large branches move, wires 'sing' Larger waves form, wave crests foam extensively
7 32-38 Moderate gale Whole trees in motion Sea heaps up - foam begins to blow in streaks
8 39-46 Fresh gale Twigs break off - progress generally impeded Waves increase visibly - foam is blown in dense streaks
9 47-54 Strong gale Slight structural damage - chimney pots removed As fresh gale
10 55-63 Whole(or full) gale Trees uprooted - considerable structural damage High waves with long overhanging crests
11 64-75 Storm Experienced at the edge of hurricanes & tornadoes - widespread damage Waves so high that ships in troughs are hidden - sea covered with streaking foam, air filled with spray
12 More than 75 Hurricane Devastation Devastation

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Return to home page Although every effort has been made to ensure the accuracy of the above information about the barometer, and the people involvd, I cannot guarentee it. If any errors are noticed, please feel fre to e-mail me at barometer@qsb.gb.com
© 2000 Peter Hanson