“We live submerged at the bottom of an ocean of the element of air which by unquestioned experiments is known to have weight”
Evangelista Torricelli 11th June 1644
By the middle of the 19th century there had been relatively few inventions which materially changed the lives of people.
Stevenson following in the footsteps of Richard Trevethic’s steam engine of 1804 had by 1814 produced the first practical steam engine. Davy invented the electric light in 1809 and the miner’s lamp in 1815, in 1831 Faraday a Dynamo.
Before the first viable machine gun invented by Richard Gatlin in 1862 and Whitehead’s torpedo of 1866 an unknown Frenchman, in 1844 Lucien Vidi perfected a mechanical barometer, the “Aneroid”, (without mercury). Essentially a mechanical device amplifying. the small expansions and contractions of an evacuated metallic capsule. Showing the resultant values on a circular analogue dial. Due to its small size and great ease of portability this invention was seized upon not just as a means of weather forecasting, a practice that was very much in its infancy but most significantly an altimeter along with its many derived applications.
Prior to the appearance of the aneroid there had been little technology introduced to the population that might be useful daily. Of course the mercury barometer had been in existence since Evangelista Torricelli’s refinement and understanding of a vacuum above a column of liquid first demonstrated by Gesparo Berti around 1640. by 1644 Torricelli had replaced the liquid (water) with mercury. A shortened version of his name became an internationally recognised unit of pressure based on the absolute scale, it is however not part of the international system of units. One Torr was intended to equal 1mm of mercury. One Torr might be expressed as 1/760 of a standard atmospherea unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa). Historically, one torr was intended to be the same as one “millimeter of mercury”. However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.
The Aneroid arguably became the first significant piece of technology, a life changer and a life saver that would be widely available to the population at large. It would not be overstated to make the comparison of Vidi’s invention to that of Harrison’s Sea watch of the 1760s. the watch enabled an accurate position of longitude.
The Aneroid apart from its application to meteorology an accurate definer of Altitude or height.
The Inquisitors of Pressure & Behaviour of Gasses
The Types of Barometer
the barometer as its name suggests is a meter that measures pressure, a term derived from the Greek Baros (pressure or weight).
Pressure may be described as an average force acting perpendicular to a surface and defined by an area, ie Lbs/Sq”.
The early 19th century gave birth to a plethora of new science and invention much of which had a direct bearing on the inseption and evolution of weather forecasting. Greater understanding of the behaviour of gases with John Dalton’s law of partial pressures, Robert Boyle’s laws in relation to density, pressure & temperature of gasses Joseph Black and his work on the latent heat of vapourisation.
There are two main types of barometer in common use, mercurial & Aneroid (without Mercury) the Former the oldest type, is essentially a column of mercury maintained in a glass tube.
The simplest type to construct, the tube approximately 35” long is held vertically, sealed end down and mercury introduced thro the open upper end, the tube is then carefully inverted, the open end positioned in a mercury bath allowing the mercury free flow in and out of the now lower open end.
If performed with care a column of mercury as measured from it’s meniscus on the top of the bath to the meniscus towards the top of the tube will be achieved.
Assuming no air or contaminants have entered the tube the height of this column will be defined by the pressure of the surrounding air acting on the surface of the mercury in the bath.
It follows therefore that should the surrounding air pressure rise more mercury will be forced up the tube, conversely should the pressure drop mercury will flow from the open end of the tube back into the bath or cistern
the consequent height of the mercury in the tube will be less.
The first patent for a mercury barometer of the type described here was granted in 1695 to Daniel Quare
A number of derivatives or variations based upon this principle followed, essentially attempts to increase accuracy Durability or portability or to produce an instrument adapted for a specific purpose. these innovations included the Angle or diagonal barometer, resembling an inverted “L” with the cistern at the bottom, the tube was canted off at shallow angle the sealed end of which ultimately achieving a vertical height for the tube of some 35”. The idea here was that the mercury responding to changes in pressure had to travel further along the sloping section of tube to reflect the same change in height that would be reflected in true vertical straight tube. This provided a wider or more open scale affording a higher degree of accuracy at the reading.
There are effectively 3 major types of mercurial Barometer, the first is laid out as a simple “U” tube, one upright of the “U” sealed with vacuum established above the mercury on the sealed side, the other upright remaining open, a scale or register affixed between the two sections
The surface of the mercury (the meniscus) will differ on both sides in height as measured vertically, one side being considerably higher than the other, the distance between the two menisci being caused by the pressure of the air acting thro the open end of the “U”
Measurement of this distance might be shown in one or more units of pressure, these might include inches, milimeters, millibars or Pascals,or less commonly Pounds per square inch, atmospheres or Torr
The Manometer is a very similar instrument, of the same layout but having both ends of the tube open to the atmosphere.
Commonly pressure may be measured as gage pressure, ie the pressure greater or lower than the prevailing ambient atmospheric pressure or as absolute pressure, the pressure to be measured being expressed as the total pressure including that of the prevailing atmospheric pressure.
The manometer may be adapted to measure both values, in order that gage pressure be measured pressure is applied to one of the open ends,the other end remaining open to the atmosphere. for absolute pressure one end remains sealed and with a vacuum established over the meniscus.
pressure is applied to the open end, the value read being the sum of the prevailing atmospheric pressure plus the value introduced.
Differential pressure ie the difference in pressure between two points may be obtained by connecting both ends of the “U” to two points, the mercury will be caused to rise or fall in either vertical tube reflecting the difference in pressure between the two points.
The second type of mercurial barometer may be termed the Fixed Cystern as applied to Marine or Kew pattern instruments. this design probably the most recognisable consists of a vertical glass tube containing mercury with a vacuum above the meniscus, the lower and open end of the tube extending into the cistern and well below the surface of the mercury.
Generally these tubes are concentrically placed to the cistern.
The plan cross sectional area of the cistern designed to be a large enough diameter to minimize the effects of zero shift. Zero shift may be defined as the distance the meniscus of the mercury may rise or fall in the cistern relative to the rise or fall of the meniscus of the mercury in the tube.
In many cases compensation for this shift might be made with a re-definition of the measuring scale, literally foreshortening the measuring units.
The cisterns of these instruments are entirely sealed save a small air way achieved within a nipple allowing a direct though very restricted vent to the atmosphere.
the flow of mercury is restricted at the base of the tube by a further narrow constriction, this in an effort to damp the effects of shipboard acceleration as the vessel rolls and pitches. The narrowness of the tube at it’s base restricted the flow of mercury by so much that readings may be historic by as much as 5 minutes.
It was for the deficiencies in this design that the Aneroid system progressively gained ground…………………………….
The third type of mercurial barometer is known as the Fortin, introduced in 1800 by Jean Nicolas Fortin, Fortin addressed within his design the issue of Zero Shift this he did with the design of his cistern. Drawn with a large bore cistern of glass, to the bottom of which was fixed a small leather bag, below which was a brass disc supported on a threaded bar terminating at it’s bottom with an adjustment knob it’s rotation causing the disc to rise or fall having a corresponding affect on the mercury meniscus within the cistern. The top of the cistern was closed with a circular plate through which and sealed into it was the base of the glass barometer tube extending downwards so that it should be well below the mercury meniscus.
The plate closing the head of the cistern was fitted with a fiducial point, a short downward pointing conical projection constructed from ivory. A fiducial point is effectively an optical reference the observation of which might easily be made thro the glass of the cistern.
The procedure for taking a reading is simple, the adjustment knob is rotated in either direction until such time as the mercury meniscus in the cistern is just touching the bottom extremity of the fiducial point. Thus is established a common datum from which all readings are made.
These instruments were improved further with the addition of a Vernier.
This instrument remained a standard for the met office for over 100 years becoming the oldest type of commercial barometer.
This type of instrument will produce a reasonably accurate reading, improvements in accuracy may be gained with a tube maintaining a highly uniform diameter over it’s length, the addition of a mirrored surface over which the readings are made will help reduce the effect of parallax a phenomena caused by viewing the meniscus away from the perpendicular. The addition of a Vernier assisting accuracy to at lease two decimal places.
In 1854 the meteorological department of the board of trade was established, this was later to become the Meteorological Office and in 1919 part of the air ministry.
Of course in order to produce useful weather forecasts not only did data gathered need to be established from standardised instruments but perhaps most importantly transmitted with speed.
It was the invention of the electric telepgraph in 1837 by Samuel FB Morse that really made all this possible.
In view of the seemingly endless loss of life particularly at sea it was determined
Under the guidance of Admiral Robert Fitzroy in association with The Royal institute to develop a standard for the Mercury Barometer, this became known as the Kew Marine Barometer.
The use of a Mercurial Barometer at sea presented a number of practical problems. The indicating fluid (mercury) especially due to it’s density made the instrument very sensitive to acceleration, pitching and rolling of the vessel creating disturbance in the height of the mercury column. Since the instrument in order for it to read correctly must be vertical it was gymbled, set within two collars with diametrically opposed pivots, the axis of which were set at 90 degrees to each other .
The tube it’s self was known to be the primary problem. In order to reduce the affects of acceleration on the reading bores became very narrow, this in turn dramatically slowed the flow of mercury between tube and cistern greatly reducing it’s sensitivity to sudden changes in pressure.
substantial improvements to the standard mercurial barometer were made Circa 1850 with the assistance of Aide developments & innovations defined the Kew Marine principles,
What is the design spec of the KEW barometer ??????????
A further derivative is known as a sympiesometer derived from the Greek sumpiedzein and metron meaning compression and measurement. this instrument was developed by Alexander Adie a patent for which was granted in 1818, it relied on the compression of hydrogen gas at the head of a glass tube sealed at the top, the hydrogen maintained in the tube above a column of coloured almond oil The hydrostatic pressure engendered in the olive oil by the atmosphere causing varying compression of the hydrogen gas, the varying level of the coloured olive oil affording a reading of atmospheric pressure at the meniscus read against a suitably calibrated register. a thermometer with a sliding scale. was provided to allow for temperature compensation.
This instrument, distantly related to the mercury barometer was originally promulgated by Robert Hooke and named a “Thermobarometer” devised and described by him to the Royal society in 1668.
Aidie’s Sympiesometer was widely trialled in the early 19th century, in particular by Fitzroy whilst on his 5 year voyage with Darwin commenced in 1831, the instrument was very well received and though primarily aimed at maritime use it suffered from a number of design flaws.
For it’s accurate operation it was vital that the hydrogen maintained it’s purity, also the volume set at ST&P be maintained.
In addition due to thermal expansion of the hydrogen readings were very much affected by temperature, in this respect a correction must be made referenced from the accompanying thermometer, this in turn determined that it could not be quickly read.
Slow evaporation of the coloured olive oil predictably & progressively corrupted the accuracy of the device. In finality it was most important the instrument not be disturbed from the vertical, moving the instrument toward the horizontal almost certainly resulting in the loss of some or all of the important hydrogen.
It is interesting to reflect upon the basic principles which notwithstanding it’s short accurate lifespan & the requirement to compensate the readings for temperature were infact sound.
The choice of Hydrogen undoubtedly made in view of it’s low density as and it’s consequentially easier compressibility, helium would have been equally efficient.
Aidie would certainly have considered the rate of evaporation of various fluids, the choice of olive oil made for it’s characteristics of modest viscosity and low evaporation.
The mercurial barometer with its many derivatives and especially after its standardisation to the Kew pattern had become a highly accurate instrument.
It had evolved not just as a measure of barometric fluctuation but also adapted for the purpose of measuring height, altitude, or as a manometer, pressure. it was other than trigonometrical calculation or measured distance the only practical path to ascertaining height or elevation.
The earliest known demonstration of this made made in 1647 by the brother in law of Blaise Pascal one Florin Perier who ascended Puy-de-Dôme
As a control Father Chastin was left in the garden below with an identical instrument. Florin Perier on his return to the garden reported that indeed the mercury level in the barometer had fallen from the value observed at the outset of the experiment from 710mm to 625mm at the highest point he achieved on the Puy-de-dome, a fall of some 12 %. Father Chastin was able to confirm that the level of mercury shown in his control barometer had remained constant at 710mm for the duration of the experiment. There followed a number of similar experiments all of which seemed to confirm the same results, essentially that air pressure fell with a rise in altitude or height. The essence of the altimeter was discovered.
The application of this instrument to the purpose of evaluating height presented very obvious difficulties, what was needed clearly was an instrument of much greater practicability.
200 years later an unknown Frenchman, Lucien Vidi who had started his working life as lawyer, becoming interested in steam engineering & the laws and effects of pressure within gasses in 1844 patented his first design for a mechanical barometer or Aneroid, (without mercury).
This device, amplified the very small expansion and contraction of a sealed and evacuated cell prevented from total collapse by internal springs. the cell resembled an oatcake with a corrugated top and bottom enabling movement similar to that of a concertina. Vidi appreciated that this sealed cell would expand as the surrounding air pressure fell and conversely contract as the pressure rose. These movements were indeed small and in order to show a pressure change upon a dial the signal from the capsule needed to be amplified, this vidi achieved with a series of levers acting upon a cross shaft finally transmitting the signal to the pointer via a fine chain or fusee with further amplification made by the action of this chain around a pulley mounted to the pointer arbor.
This in simple terms is the essence of a device that was to revolutionise meterology, mining, civil construction & geographical survey.
It was to become essential in particular to polar and alpine explores and arguably of most importance to mariners. Vidi had invented the “had to have” device, something with far more value than just a gadget, a life saver of immense practical importance.
There exists one other practical method of determining barometric pressure, the Hypsometer, a device used to measure height or elevation.
3 types exist,the first using lasers in the way of a range finder, the second a mechanical device based on trigonometrical calculation the third and certainly the most historically significant the barometric variant.Sometimes referred to as a thermometrical barometer this instrument was invented by the well known Victorian scientist F.J.H.Wollaston, he had submitted two papers to the Royal society in 1817 and later in 1820.
In essence he described an instrument that accurately measured the boiling point of distilled water, he asserted that the boiling point of water would alter as the ambient air pressure altered, if therefore the temperature could be accurately determined it would follow that this in turn could be related to barometric pressure and would serve as a good guide to altitude.
The first version of this instrument consisted of a water reservoir above which was held a long thermometer contained within a box wood sleeve along the length of which was a slot or cut out thro which the the mercury thread may be seen.An applied paper scale was marked in degrees Fahrenheit and thousands of feet divided to 100.
A second and more refined version here illustrated shows the instrument as protected within a short heavy leather case, the main body of the instrument enclosed within a cylindrical lacquered oxidised brass wind shield, this may be drawn apart the instrument withdrawn, the telescopic thermometer shroud removed and re-fitted by it’s other end, it may then be extended as the draws of a telescope the thermometer inserted by it’s top.
The spirit burner in the base is supplied with a screw down top to prevent any fuel leakage, this is removed, the wick lit, one of the two thermometers supplied in lacquered brass sleeves fitted into the aperture at the top of the extended telescopic draws, the water container filled and the whole assembly placed back within it’s windshield.
At the point the water boils, established when the temperature fails to rise further a reading may be taken.
These instruments were not simply stand alone, though consulted regularly they served as an important calibration tool for any of the aneroid types that were carried on these expeditions. The aneroid as a mechanical device being prone to inaccuracy due to low temperature shock and vibration not to mention the poor performance of lubricants .
This instrument proved to be uniquely useful to polar and Alpine explorers it was certainly used in Scott’s Antarctic expedition of 1901and 1914 – 17, a similar instrument was also used during Livingstone’s expedition of N.Africa 1853 – 56
It remains unclear as to who the maker of these instruments was, it was common practice at this time for retailers to order such instruments from a single specialist manufacturer but marked with the business name of the seller. In this instance this instrument is marked Hicks, the instrument taken on Scott’s expedition of 1901 is near identical but marked casella.
This instrument has the initials W.J.A struck into the lid of the leather carry case.
These instruments were certainly in use up to and including WW11, this example made by Lufft was issued to U Boats and was used for re-calibrating on board barometers and barographs used in weather reporting boats.
As E.J.Dent remarked within the pages of his Treatise on the Aneroid written in 1849
“it seems a matter of surprise that so many distinguished men should have devoted so much time to improve the old barometer without even an attempt to supply it’s place by a vacuum vase as the latter renders the instrument incalculably more useful from it’s portability”
The vacuum vase referred to was proposed by one N.J.Conte, professor of the aerostatical school at Meudon, Paris, France.
Conte’s proposition was made in the Bulletin des sciences year 6 (1798), essentially this described a small vessel, perhaps the size of a pocket watch, in section roughly describing an elipse.
the lower half to be formed in thick walled iron, the upper part in thin walled steel. Carefully seated so as to provide a seal of high integrity, tensioned with internal springs the air was then withdrawn, the springs employed to provide a counterbalance to the action of atmospheric pressure now applied across the outer surface area of the whole vessel. Of course it was anticipated by Conte that the lower heavy iron part would completely resist any deformation caused by pressure,it was intended that the upper part of thin steel would rise or fall with external pressure change.
As dent correctly observes it is little wonder that this design rendered the idea useless.
Of all the forms that might have been chosen for the upper thin walled steel part an arch or dome must be the worst as such structures are designed specifically to resist load or weight the material of the structure designed to accept compression and not bending as we see daily in bridges or arches.
It was intended that The resultant expansion and contraction of the thin walled steel upper part might be shown upon a dial by a pointer.
Conte though clearly the originator of the Aneroid is said to have become disillusioned with his invention since it seemed more eager to respond to changes of temperature than pressure.
Here then in 1798 was the germ of an idea that due to poor design became a fail
46 years later enter an unknown, one Lucien Vidi, a failed French lawyer with an interest in steam and engineering, he had shown a particular interest in manometers and from that one might assume pressure in steam and gasses. Interestingly his name in French might be mistaken for vacuum, the French being “Vide”
In 1844 the first patent was taken out by Pierre Fontainemoreau in England on behalf of Vidi for what was to be called an Aneroid, variously translated this means “without mercury”
Descibed as a small lenticular box of copper construction and fitted with internal springs.
This box had the air inside removed achieving a near vacuum, the springs employed to prevent it’s complete collapse.
L’Année scientifique et industrielle, Volume 11 (1867)
At the beginning of April 1866 French science lost a distinguished man Lucien Vidi the inventor of the aneroid barometer He was still only sixty-one Unexpected death surprised in the middle of important research
Born in Nantes in 1805 Vidi was destined to first in the ecclesiastical state but the practical mechanics work offered more attraction to this active man and stirring
It is in 1844 that he took his first patent for a new metal barometer called by him aneroid This word means without air but it is a barbarism which would mean rather like a man than airless The principle of this instrument is similar to that of the bladder crest It is exposed to the pressure of the outside air a hollow vessel whose form has an unequal resistance such as that of a sphere flattened in one has evacuated Some of the parts of this vase then closer together than the others under the effort of the atmospheric and this displacement effect is transmitted using screws or gears to a needle that travels a dial
In the Vidi aneroid barometer, the vessel is a small very-walled copper lenticular box whose spacing is held by an internal spring which gives way under the pressure of the air when this pressure increases and which relaxes when it decreases The lower wall is fixed the upper one controls a transmission of movement which makes work a needle
Vidi made execute its first barometers by M Rédier but soon it took charge He was passionate about his aneroid Having devoured all his patrimony in trials of all kinds, he found himself one day very much in danger of completing his work when a friend came to his aid by putting at his disposal all the sums he could he needed a great fortune just reward of so much persistence But many lawsuits raised by his invention came to disturb his peaceful life The struggles he had to support had made him misanthropic about the end of his days His lawsuits were directed against Mr. Bourdon who in 1849 had taken a patent for a metal barometer whose essential organ was an empty metal tube and flattened Vidi saw in it a counterfeit of his aneroids The first lawsuit filed by Vidi in 1852 was lost by him but in 1858 the court recognized its priority with respect to M Bourdon and defended the sale of the barometers with tubes until the expiration of the patents of Vidi
Vidi had the mania of hydrotherapy pushed to excess he was taking sea baths in winter it is a bath taken in these conditions who said he was killed
With the filing of Vidi’s patent in 1844 begun a catalogue of trials and tribulations for the inventor, law suits followed in defence of the patent primarily against Bourdon who adapted his flattened tube as a different manifestation of the Vidi “Box” or what we now term a capsule, another court case brought by Redier who had been commissioned by Vidi to make the first batch of Aneroid Barometers. Vidi had asserted that the made instruments were not of sufficiently good standard and withheld payment.
All this turmoil and scandal did nothing to help promote this ground breaking invention in France or across the continent.
It went to a well-respected London Based instrument maker Dent who on seeing the instrument easily recognised the potential and begun to import Vidi made instruments into the UK. Needless to say they sold very quickly, soon coming to the attention of the Royal Navy civilians and formal organisations alike.
So the first and very arguably the most significant of these instruments appeared on general sale, often presented in desk cases of many different designs and finishes, essentially two main designs emerged, a sloping case with a glazed viewing port or a flat box type case , the rear fitted with an extending easel stand and or a sliding plate that may enable the instrument to be wall hung.
These instruments were manufactured with a number of different dial configurations the vast majority calibrated in Inches Hg but with varying ranges,some with the addition of semi-circular thermometers, the most common being 28 – 31 though wider ranges are seen with lowest pressures around 23”Hg.
It would appear that almost instantly some of these instruments were manufactured to special order for specific application reflected in the range and it’s division .
Many disciplines quickly recognised the obvious benefits of the instrument.
Maritime, the obvious advantages of a small compact instrument largely unaffected by the pitching and rolling of a ship or yacht, up to this point barometers had been of the “Stick” type and it’s variants , this required a dedicated bulkhead space where it could be supported in a gimballed mount, the aneroid contained in a small wooden box could be easily stowed or made constantly available on a chart table.
In addition the Aneroid design is far less likely to become damaged than the traditional glass tube.
Heart of the Aneroid Barometer or Altimeter
Referred to originally as a Vacuum Vase by Conte and then a lenticular shaped box By Vidi, the latter manifest as a hollow disc comprising at least 5 externally visible parts, the circumference comprising a band placed between the top and bottom plates and completely soldered around.
I will refer to this component as a Cell, in other words a self-contained autonomous unit though it has also been described as a Diaphragm By Negretti & Zambra, as part of it’s function it does transmit load but the term does not define the other essential characteristics.
The bottom with centre mounted fixing point either as a threaded projection or as a tapped bore , this enabled the unit to be rigidly fixed to a chassis or other anker point of the instrument.In rare instances I have observed the capsule soldered directly to the chassis of the instrument. Not a good idea since the application of heat would very likely compromise the integrity of previously soldered joints. Where the solder is softened the presence of very low pressure (VLP) may cause the joint to blow or fail.
Looking at the earliest capsules, largely constructed from two spun corrugated copper discs, once constructed air is withdrawn via a vacuum pump through a nipple or extension at the circumference of the cell, once the desired VLP has been achieved the nipple is crimped and soldered.
The top and bottom surfaces now achieve a dished or convex appearance caused by atmospheric pressure acting on these surfaces. Of course that Atmospheric pressure acts equally all round the capsule, the circumference remaining unaffected by virtue of it’s shape capable of resisting high loadings being in compression.
So we have a hollow cell with low pressure air inside, I say low pressure and not a vacuum because the vacuum pumps of the day were not capable of drawing the pressure down to what might be described as a vacuum. In later cells this state of VLP was intentional and I will talk about that later.
The object of this exercise is to produce a cell that responds to changes in barometric pressure to bring that about there needs to be an equal and opposite force applied to the cell, ie equal to and opposite to the force of atmospheric pressure causing it to deform inwards.
This was achieved in two ways, either by the fitting of internal springs so acting from the inside pushing out or by the attachment of an external spring drawing the top surface way from the bottom fixed to the instrument.
Thus if the springs internal or external achieve an equilibrium or balance to the force of atmospheric pressure should that atmospheric pressure change there will be deflection of the cell, in the case of rising pressure the top will be forced closer to the bottom and in the case of falling pressure the top will move further away from the bottom.
Mean atmospheric pressure as measured at sea level is 29.92”Hg or 1,013Mb, generally we see pressure changes in the order of 2 ½” Hg 0r 84.65 Mb, somewhere between 28 and 30.5”Hg or about 8% of atmospheric pressure.
This is important because it translates into only a very small amount of expansion and contraction of the capsule about 0.013” across the 28- 31 range.
In order to transalate this small movement into a much larger one as seen by the movement of the pointer across the barometer
A number of different designs are known, the shape and composition together with methods of making evolving over time. These will be covered later.