JOHN DALTON (1766-1844)

1801 England

‘The total pressure of a mixture of gases is the sum of the partial pressures exerted by each of the gases in the mixture’

Partial pressures of gases:
Dalton stated that the pressure of a mixture of gases is equal to the sum of the pressures of the gases in the mixture. On heating gases they expand and he realised that each gas acts independently of the other.

Each gas in a mixture of gases exerts a pressure, which is equal to the pressure it would exert if it were present alone in the container; this pressure is called partial pressure.

Dalton’s law of partial pressures contributed to the development of the kinetic theory of gases.

His meteorological observations confirmed the cause of rain to be a fall in temperature, not pressure and he discovered the ‘dew point’ and that the behaviour of water vapour is consistent with that of other gases.

He showed that a gas could dissolve in water or diffuse through solid objects.

Graph demonstrating the varying solubility of gases

The varying solubility of gases

Further to this, his experiments on determining the solubility of gases in water, which, unexpectedly for Dalton, showed that each gas differed in its solubility, led him to speculate that perhaps the gases were composed of different ‘atoms’, or indivisible particles, which each had different masses.
On further examination of his thesis, he realised that not only would it explain the different solubility of gases in water, but would also account for the ‘conservation of mass’ observed during chemical reactions – as well as the combinations into which elements apparently entered when forming compounds – because the atoms were simply ‘rearranging’ themselves and not being created or destroyed.

In his experiments, he observed that pure oxygen will not absorb as much water vapour as pure nitrogen – his conclusion was that oxygen atoms were bigger and heavier than nitrogen atoms.

‘ Why does not water admit its bulk of every kind of gas alike? …. I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases ’

In a paper read to the Manchester Society on 21 October 1803, Dalton went further,

‘ An inquiry into the relative weight of the ultimate particles of bodies is a subject as far as I know, entirely new; I have lately been prosecuting this enquiry with remarkable success ’

Dalton described how he had arrived at different weights for the basic units of each elemental gas – in other words the weight of their atoms, or atomic weight.

Dalton had noticed that when elements combine to make a compound, they always did so in fixed proportions and went on to argue that the atoms of each element combined to make compounds in very simple ratios, and so the weight of each atom could be worked out by the weight of each element involved in a compound – the idea of the Law of Multiple Proportions.

When oxygen and hydrogen combined to make water, 8 grammes of oxygen was used for every 1 gramme of hydrogen. If oxygen consisted of large numbers of identical oxygen atoms and hydrogen large numbers of hydrogen atoms, all identical, and the formation of water from oxygen and hydrogen involved the two kinds of atoms colliding and sticking to make large numbers of particles of water (molecules) – then as water has an identity as distinctive as either hydrogen or oxygen, it followed that water molecules are all identical, made of a fixed number of oxygen atoms and a fixed number of hydrogen atoms.

Dalton realised that hydrogen was the lightest gas, and so he assigned it an atomic weight of 1. Because of the weight of oxygen that combined with hydrogen in water, he first assigned oxygen an atomic weight of 8.

There was a basic flaw in Dalton’s method, because he did not realise that atoms of the same element can combine. He assumed that a compound of atoms, a molecule, had only one atom of each element. It was not until Italian scientist AMADEO AVOGADRO’s idea of using molecular proportions was introduced that he would be able to calculate atomic weights correctly.

In his book of 1808, ‘A New System of Chemical Philosophy’ he summarised his beliefs based on key principles: atoms of the same element are identical; distinct elements have distinct atoms; atoms are neither created nor destroyed; everything is made up of atoms; a chemical change is simply the reshuffling of atoms; and compounds are made up of atoms from the relevant elements. He published a table of known atoms and their weights, (although some of these were slightly wrong), based on hydrogen having a mass of one.

Nevertheless, the basic idea of Dalton’s atomic theory – that each element has its own unique sized atoms – has proved to be resoundingly correct.

If oxygen atoms all had a certain weight which is unique to oxygen and hydrogen atoms all had a certain weight that was unique to hydrogen, then a fixed number of oxygen atoms and a fixed number of hydrogen atoms combined to form a fixed weight of water molecules. Each water molecule must therefore contain the same weight of oxygen atoms relative to hydrogen atoms.

Here then is the reason for the ‘law of fixed proportions’. It is irrelevant how much water is involved – the same factors always hold – the oxygen atoms in a single water molecule weigh 8 times as much as the hydrogen atoms.

Dalton wrongly assumed that elements would combine in one-to-one ratios as a base principle, only converting into ‘multiple proportions’ (for example from carbon monoxide, CO, to carbon dioxide, CO2) under certain conditions. Each water molecule (H2O) actually contains two atoms of hydrogen and one atom of oxygen. An oxygen atom is actually 16 times as heavy as a hydrogen atom. This does not affect Dalton’s reasoning.

The law of fixed proportions holds because a compound consists of a large number of identical molecules, each made of a fixed number of atoms of each component element.

Although the debate over the validity of Dalton’s thesis continued for decades, the foundation for the study of modern atomic theory had been laid and with ongoing refinement was gradually accepted.

A_New_System_of_Chemical_Philosophy - DALTON's original outline

A_New_System_of_Chemical_Philosophy

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THOMAS YOUNG (1773-1829)

1801 – England

‘Interference between waves can be constructive or destructive’

Huygens‘ wave theory was neglected for more than a hundred years until it was revived by Young in the opening years of the nineteenth century. Young rejected Newton‘s view that if light consisted of waves it would not travel in a straight line and therefore sharp shadows would not be possible. He said that if the wavelength of light was extremely small, light would not spread around corners and shadows would appear sharp. His principle of interference provided strong evidence in support of the wave theory.

Young’s principle advanced the wave theory of light of CHRISTIAAN HUYGENS. Further advances came from EINSTEIN and PLANCK.

In Young’s double slit experiment a beam of sunlight is allowed to enter a darkened room through a pinhole. The beam is then passed through two closely spaced small slits in a cardboard screen. You would expect to see two bright lights on a screen placed behind the slits. Instead a series of alternate light and dark stripes are observed, known as interference fringes, produced when one wave of light interferes with another wave of light.

Two identical waves traveling together either reinforce each other (constructive interference) or cancel each other out (destructive interference). This effect is similar to the pattern produced when two stones are thrown into a pool of water.

portrait of THOMAS YOUNG ©

THOMAS YOUNG

The mathematical explanation of this effect was provided by AUGUSTIN FRESNEL (1788-1827). The wave theory was further expanded by EINSTEIN in 1905 when he showed that light is transmitted as photons.

Light, an electromagnetic radiation, is transported in photons that are guided along their path by waves. This is known as ‘wave-particle duality’.

The current view of the nature of light is based on quantum theory.

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LUKE HOWARD (1772-1864)

1802 – England

‘All types of clouds can be categorised into three basic families’

luke howard cloud chart

  • High clouds (their bases above 6km) – CIRRUS (hair-like)

  • a. Cirrus
  • b. Cirrocumulus
  • c. Cirrostratus

 

  • Middle clouds (between 2 and 6km) – CUMULUS (puffs)

Nimbostratus cloud type

Nimbostratus

 

    • Low clouds (below 2km) – STRATUS (layers)

  • a. Stratocumulus
  • b. Stratus
  • c. Cumulus

 

Intermediate and compound types are cumulocirrostratus or nimbus (the rain cloud).

Clouds that stretch through the three altitude bands are cumulonimbus.

Portrait of Luke Howard

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JOSEPH LOUIS GAY-LUSSAC (1778-1850)

1808 – France

‘Volumes of gases which combine or which are produced in chemical reactions are always in the ratio of small whole numbers’

One volume of nitrogen and three volumes of hydrogen produce two volumes of ammonia. These volumes are in the whole number ratio of 1:3:2

N2 + 3H2 ↔ 2NH3

Along with his compatriot Louis Thenard, Gay-Lussac proved LAVOISIER’s assumption, that all acids had to contain oxygen, to be wrong.

portrait of GAY-LUSSAC ©

GAY-LUSSAC

Gay-Lussac re-examined JACQUES CHARLES’ unpublished and little known work describing the effect that the volume of a gas at constant pressure is directly proportional to temperature and ensured that Charles received due credit for his discovery.

Alongside JOHN DALTON, Gay-Lussac concluded that once pressure was kept fixed, near zero degrees Celsius all gases increased in volume by 1/273 the original value for every degree Celsius rise in temperature. At 10degrees, the volume would become 283/273 of its original value and at – 10degrees it would be 263/273 of that same original value. He extended this relation by showing that when volume was kept fixed, gas would increase or decrease the pressure exerted on the outside of the gas container by the same 1/273 factor when temperature was shifted by a degree Celsius. This did not depend upon the gas being studied and hinted at a deep connection shared by all gases. If the volume of a gas at fixed pressure decreased by 1/273 for every 1degree drop, it would reach zero volume at -273degrees Celsius. The same was true for pressure at fixed volume. That had to be the end of the scale, the lowest possible temperature one could reach. Absolute zero.

In an 1807 gas-experiment, Gay-Lussac took a large container with a removable divider down the middle and filled half with gas and made the other half a vacuüm. When the divider was suddenly removed, the gas quickly filled the whole container. According to caloric theory, temperature was a measure of the concentration of caloric fluid and removal of the divider should have led to a drop in temperature because the fluid was spread out over a greater volume without any loss of caloric fluid. (The same amount of fluid in a larger container means lower concentration).
Evidence linking heat to mechanical energy accumulated. Expenditure of the latter seemed to lead to the former.

Gay-Lussac was an experimentalist and his law was based on extensive experiments. The explanation of why gases combine in this way came from AVOGADRO.

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JOHN DALTON

1808 – Manchester, England

‘All matter is made up of atoms, which cannot be created, destroyed or divided. Atoms of one element are identical but different from those of other elements. All chemical change is the result of combination or separation of atoms’

Dalton struggled to accept the theory of GAY-LUSSAC because he believed, as a base case, that gases would seek to combine in a one atom to one atom ratio (hence he believed the formula of water to be HO not H2O). Anything else would contradict Dalton’s theory on the indivisibility of the atom, which he was not prepared to accept.

The reason for the confusion was that at the time the idea of the molecule was not understood.
Dalton believed that in nature all elementary gases consisted of indivisible atoms, which is true for example of the inert gases. The other gases, however, exist in their simplest form in combinations of atoms called molecules. In the case of hydrogen and oxygen, for example, their molecules are made up of two atoms, described as H2 and O2 respectively.

Gay-Lussac examined various substances in which two elements form more than one type of compound and concluded that if two elements A and B combine to form more than one compound, the different masses of A that combine with a fixed mass of B are in a simple whole number ratio. This is the law of multiple proportions.

AVOGADRO’s comprehension of molecules helped to reconcile Gay-Lussac’s ratios with Dalton’s theories on the atom.

Gay-Lussac’s ratio for water could be explained by two molecules of hydrogen (four ‘atoms’) combining with one molecule of oxygen (two ‘atoms’) to result in two molecules of water (2H2O).

2H2 + O2 ↔ 2H2O

When Dalton had considered water, he could not understand how one atom of hydrogen could divide itself (thereby undermining his indivisibility of the atom theory) to form two particles of water. The answer proposed by Avogadro was that oxygen existed in molecules of two and therefore the atom did not divide itself at all.

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AMEDEO AVOGADRO (1776-1856)

1811 – Italy

‘Equal volumes of all gases at the same temperature and pressure contain the same number of molecules’

In 1811, when Avogadro proposed his HYPOTHESIS, very little was known about atoms and molecules. Avogadro claimed that the same volume of any gas under identical conditions would always contain the same number of fundamental particles, or molecules. A litre of hydrogen would contain exactly the same number of molecules as a litre of oxygen or a litre of carbon dioxide.

Drawing of AVOGADRO ©

In 1814 ANDRE AMPERE was credited with discovering that if a gas consisted of a single element, its atoms could clump in pairs. The molecules of oxygen consisted of pairs of oxygen atoms, and the molecules of chlorine, pairs of chlorine atoms.
Diatomic gases possess a total of six degrees of simple freedom per molecule that are related to atomic motion.

This provides a way of comparing the weights of different molecules. It was only necessary to weigh equal volumes of different gases and compare them. This would be exactly the same as comparing the weights of the individual molecules of each gas.

Avogadro realised that GAY-LUSSAC‘s law provided a way of proving that an atom and a molecule are not the same. He suggested that the particles (molecules) of which nitrogen gas is composed consist of two atoms, thus the molecule of nitrogen is N2. When one volume (one molecule) of nitrogen combines with three volumes (three molecules) of hydrogen, two volumes (two molecules) of ammonia, NH3, are produced.

N2 + 3H2 ↔ 2NH3

However, the idea of a molecule consisting of two or more atoms bound together was not understood at that time.

Avogadro’s law was forgotten until 1860 when the Italian chemist STANISLAO CANNIZZARO (1826-1910) explained the necessity of distinguishing between atoms and molecules.

Avogadro’s constant
From Avogadro’s law it can be deduced that the same number of molecules of all gases at the same temperature and pressure should have the same volume. This number has been determined experimentally: it’s value is 6.022 1367(36) × 1023AVOGADRO’S NUMBER

Avogadro's_number_in_e_notation

That at the same temperature and pressure, equal volumes of all gases have the same number of molecules allows a simple calculation for the combining ratios of all gases – by measuring their percentages by volume in any compound. This in turn facilitates simple calculation of the relative atomic masses of the elements of which it is composed.

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WILLIAM PROUT (1785-1850)

1815 – UK

‘Atoms are not the smallest thing’

After ANTOINE LAVOISIER had compiled his list of the then known elements, another 32 were added in the years following his death. Fifty kinds of fundamental building blocks for matter seemed excessive. In 1815 Prout, using AVOGADRO’s method of comparing the relative densities and weights of gases, proposed that all atoms appeared to have weights that were exact multiples of the weight of the lightest atom, hydrogen, and that the different atomic weights of elements are whole-number multiples of the atomic weight of hydrogen (Prout’s hypothesis).

Portreait of William Prout (c) The University of Edinburgh Fine Art Collection; Supplied by The Public Catalogue Foundation

WILLIAM PROUT

He took this as proof that all atoms were actually made from hydrogen atoms and the idea was adopted as atomic theory and used for later investigations of atomic weights and the classification of the elements.

If all atoms are made from atoms of hydrogen, then it could be possible to transform an atom of one element into an atom of another.
If atoms had been assembled from other things, then they themselves could not be the smallest things in creation.

Apart from the method of weighing atoms being controversial, there are exceptions to the rule. Chlorine is 35.5 times as heavy as hydrogen.

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HANS CHRISTIAN OERSTED (1777-1851)

1820 – Denmark

‘Electric current produces a magnetic field’

drawn portrait of HANS CHRISTIAN OERSTED ©

Oersted discovered that an electric current could make the needle of a magnetic compass swivel. It was the first indication of a link between these two natural forces. Although Oersted discovered electromagnetism he did little about it. This task was left to AMPERE and FARADAY.

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HEINRICH WILHELM MATTHIAS OLBERS (1758-1840)

1823 – Germany

‘Why is the sky dark at night?’

This question puzzled astronomers for centuries and no, the answer is not because the Sun is on the other side of the planet.

Olbers pointed out that if there were an infinite number of stars evenly distributed in space, the night sky should be uniformly bright. He believed that the darkness of the night sky was due to the adsorption of light by interstellar space.

This is wrong.

Heinrich-Wilhelm-Matthias-Olbers ©

OLBERS

Olbers’ question remained a paradox until 1929 when it was discovered that the galaxies are moving away from us and the universe is expanding. The distant galaxies are moving away so fast that the intensity of light we receive from them is diminished.

diagram explaining reduced light intensity as the observer travels further from the source

What is light intensity?

In addition, this light is shifted towards the red end of the spectrum. These two effects significantly reduce the light we receive from distant galaxies, leaving only the nearby stars, which we see as points of light in a darkened sky.

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JOSEF VON FRAUNHOFER (1787-1826)

1823 – Germany

‘The spectroscope’

A significant improvement on the apparatus used by Newton. Sunlight, instead of passing through a pinhole before striking a prism, is passed through a long thin slit in a metal plate. This creates a long ribbon-like spectrum, which may be scanned from end to end with a microscope.

image of the visible portion of the electromagnetic spectrum showing a series of dark fraunhofer lines

Cutting across the ribbon of rainbow colours are thin black lines. The lines are present even when a diffraction grating is used instead of a prism, proving that the lines are not produced by the material of a prism, but are inherent in sunlight.

An equivalent way of describing colours is as light waves of different sizes.
The wavelength of light is fantastically small, on average about a thousandth of a millimeter, with the wavelength of red light being about twice as long as that of blue light.

Fraunhofer’s black lines correspond to missing wavelengths of light.

By 1823 Fraunhofer had measured the positions of 574 spectral lines, labeling the most prominent ones with the letters of the alphabet. The lines labeled with the letters ‘H’ and ‘K’ correspond to light at a wavelength of 0.3968 thousandths of a millimeter and 0.3933 thousandths of a millimeter, respectively. The lines are present in the spectrum of light from stars, usually in different combinations.

Fraunhofer died early at the age of 39 and it was left to the German GUSTAV KIRCHOFF to make the breakthrough that explained their significance.

Astronomers today know the wavelengths of more than 25,000 ‘Fraunhofer lines’.

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NICOLAS SADI CARNOT (1796-1832)

1824 – France

‘The Carnot cycle is the most efficient cycle for operating a reversible heat engine’

It illustrates the principle that the efficiency of a heat engine depends on the temperature range through which it works.

The cycle has a four-stage reversible sequence:

adiabatic compression and isothermal expansion at high temperature; adiabatic expansion and isothermal compression at low temperature.

( ADIABATIC: – no heat flows into or out of a system; ISOTHERMAL: – at a constant temperature )

Carnot suggested that the puissance motrice (motive power, by which he meant work or energy) of a heat engine was derived from the fall of heat from a higher to a lower temperature.

Carnot was the first to grasp the principles that later became known as the second law of thermodynamics.

By the time of Carnot’s death it had become clear there was no such thing as a calorific fluid ; heat is a form of energy, one of many, and the sum of all forms of energy in an isolated system is conserved. This has come to be known as the first law of thermodynamics.

In the case of a steam engine, the heat taken in at the boiler is not equal to the heat removed at the condenser. The work done by the ideal engine is the difference between the two. The first modern experiment proving the first law of thermodynamics was performed by a student of John Dalton’s, JAMES JOULE.

 

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GEORG SIMON OHM (1789-1854)

1827 – Germany

‘The electric current in a conductor is proportional to the potential difference’

In equation form, V = IR, where V is the potential difference, I is the current and R is a constant called resistance.

greek symbol capital ohm (480 x 480)

Ohm’s law links voltage (potential difference) with current and resistance and the scientists VOLTA, AMPERE and OHM.

Ohm is now honoured by having the unit of electrical resistance named after him.
If we use units of VI and R, Ohm’s law can be written in units as:

volts = ampere × ohm

photograph of george simon ohm © + diagram of simple electric circuit

GEORG OHM


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ANDRE MARIE AMPERE (1775-1836)

1827 – France

‘Two current-carrying wires attract each other if their currents are in the same direction, but repel each other if their currents are opposite. The force of attraction or repulsion (magnetic force) is directly proportional to the product of the strengths of the currents and inversely proportional to the square of the distance between them’

portrait of ANDRE AMPERE ©

ANDRE AMPERE

Another addition to the succession of ‘inverse-square’ laws begun with NEWTON’s law of universal gravitation.
Ampere had noted that two magnets could affect each other and wondered, given the similarities between electricity and magnetism, what effect two currents would have upon each other. Beginning with electricity run in two parallel wires, he observed that if the currents ran in the same direction, the wires were attracted to each other and if they ran in opposite directions they were repelled.

He experimented with other shapes of wires and generalised that the magnetic effect produced by passing a current in an electric wire is the result of the circular motion of that current. The effect is increased when the wire is coiled. When a bar of soft iron is placed in the coil it becomes a magnet. This is the solenoid, used in devices where mechanical motion is required.

Ampere exploited OERSTED’s work, devising a galvanometer which measured electric current flow via the degree of deflection upon its magnetic needle.

He attempted to interpret all his results mathematically in a bid to find an encompassing explanation for what later became referred to as electromagnetism (Ampere had at that time christened it electrodynamics), resulting in his 1827 definition.

Ampere’s name is commemorated in the SI unit of electric current, the ampere.

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ROBERT BROWN (1773-1858)

1827 – UK

‘Tiny solid particles suspended in a fluid are in continuous random motion’

This motion is caused by constant collisions between the suspended particles and the fluid molecules.

In 1905 EINSTEIN studied Brownian motion and used it to calculate the approximate mass and size of atoms and molecules.

Robert Brown (1773-1858), British botanist. Brown is most famous for his 1827 observation of erratic motion by pollen grains in water. This was named Brownian motion.In 1877, Desaulx recognised that the motion is caused by the pollen colliding with water molecules. This meant that Brownian motion was the first directly observable evidence for the existence of molecules. Brown spent years working on plant taxonomy, establishing the classification of two major divisions of plants, the gymnosperms and the angiosperms. He also observed an essential part of living cells, which he named the nucleus (1831) &copy:

ROBERT BROWN

Brown is also remembered for discovering a small body within cells, which he named the nucleus (from the Latin for ‘little nut’). Plant cells were discovered by HOOKE.

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EVARISTE GALOIS (1811- 32)

1832 – France

‘The study of solutions of some equations and how different solutions are related to each other’

Bust depicting Evariste Galois©

Or, the study of certain groups, known as Galois groups, that can be associated with polynomial equations.
Whether or not the solutions to an equation can be written down using rational functions and square roots, cube roots, etc., depends on certain group-theoretic properties of Galois groups.

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CARL GAUSS (1777-1855)

1832 – Germany

‘The electrical flux through a closed surface is proportional to the sum of the electric charges within the surface’

 Portrait of GAUSS ©

GAUSS

An electric field may be pictured by drawing lines of force. The field is stronger where these lines crowd together, weaker where they are far apart. Electrical flux is a measure of the number of electric field lines passing through an area.

Gauss’ law describes the relationship between electric charge and electric field. It is an elegant restatement of COULOMB‘s law.

  

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MICHAEL FARADAY (1791-1869)

1831 – England

‘A changing magnetic field around a conductor produces an electric current in the conductor. The size of the voltage is proportional to the rate of change of the magnetic field’

portrait drawing of MICHAEL FARADAY English chemist and physicist (British Library) (1791-1867)

This phenomenon is called ‘electromagnetic induction’ and the current produced ‘induced current’. Induction is the basis of the electric generator and motor.

Faraday developed HANS CHRISTIAN OERSTED’s 1820 discovery that electric current could deflect a compass needle. In his experiment Faraday wrapped two coils of insulated wire around opposite sides of an iron ring. One coil was connected to a battery, the other to a wire under which lay a magnetic compass needle. He anticipated that if he passed a current through the first wire it would establish a field in the ring that would induce a current in the second wire. He observed no effect when the current was steady but when he turned the current on and off he noticed the needle moving. He surmised that whenever the current in the first coil changed, current was induced in the second. To test this concept he slipped a magnet in and out of a coil of wire. While the magnet was moving the compass needle registered a current, as he pushed it in it moved one way, as he pulled it out the needle moved in the opposite direction. This was the first production of electricity by non-chemical means.

In 1831, by rotating a copper disc between the poles of a magnet, Faraday was able to produce a steady electric current. This was the world’s first dynamo.

NEWTON, with his concept of gravity, had introduced the idea of an invisible force that exerted its effect through empty space, but the idea of ‘action-at-a-distance’ was rejected by an increasing number of scientists in the early nineteenth century. By 1830, THOMAS YOUNG and AUGUSTIN FRESNEL had shown that light did not travel as particles, as Newton had said, but as waves or vibrations. But if this was so, what was vibrating? To answer this, scientists came up with the idea of a weightless matter, or ‘aether’.

Faraday had rejected the concept of electricity as a ‘fluid’ and instead visualised its ‘fields’ with lines of force at their edges – the lines of force demonstrated by the pattern of iron fillings around a magnet. This meant that action at a distance simply did not happen, but things moved only when they encountered these lines of force. He believed that magnetism was also induced by fields of force and that it could interrelate with electricity because the respective fields cut across each other. Proving this to be true by producing an electric current via magnetism, Faraday had demonstrated electromagnetic induction.

When Faraday was discovering electromagnetic induction he did so in the guise of a natural philosopher. Physics, as a branch of science, was yet to be given a name.

The Russian physicist HEINRICH LENZ (1804- 65) extended Faraday’s work when in 1833 he suggested that ‘the changing magnetic field surrounding a conductor gives rise to an electric current whose own magnetic field tends to oppose it.’ This is now known as Lenz’s law. This law is in fact LE CHATELIER‘s principle when applied to the interactions of currents and magnetic fields.

Fluctuating_Electromagnetic_Fields_and_EM_Waves

Fluctuating Electromagnetic Fields and EM Waves

It took a Scottish mathematician by the name of JAMES CLERK MAXWELL to provide a mathematical interpretation of Faraday’s work on electromagnetism.

Describing the complex interplay of electric and magnetic fields, he was able to conclude mathematically that electromagnetic waves move at the speed of light and that light is just one form of electromagnetic wave.
This led to the understanding of light and radiant heat as moving variations in electromagnetic fields. These moving fields have become known collectively as radiation.

Faraday continued to investigate the idea that the natural forces of electricity, magnetism, light and even gravity are somehow ‘united’, and to develop the idea of fields of force. He focused on how light and gravity relate to electromagnetism.
After conducting experiments using transparent substances, he tried a piece of heavy lead glass, which led to the discovery of the ‘Faraday Effect’ in 1845 and proved that polarised light may be affected by a magnet. This opened the way for enquiries into the complete spectrum of electromagnetic radiation.

In 1888 the German physicist HEINRICH HERTZ confirmed the existence of electromagnetic waves – in this case radio waves – traveling at the speed of light.

The unit of capacitance, farad (F) is named in honour of Faraday.

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Faraday Lecture -‘The chemical history of a candle’
Faraday as a discoverer

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THOMAS GRAHAM (1805- 69)

1833 – UK

‘Under the same conditions, the rate of diffusion of a gas is inversely proportional to the square root of its density’

For example, hydrogen diffuses four times as fast as oxygen under the same conditions of temperature and pressure.

Gases have no fixed volume; they expand to fill the entire volume of their container. This spreading of gas particles is called diffusion. The lightest gases diffuse most rapidly.

Graham is referred to as the father of colloid chemistry. In 1854 he invented the process of dialysis, which is based on the principle that some material will diffuse across a semi-permeable membrane and some material will not.

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CHARLES BABBAGE (1791-1871) ADA LOVELACE (1815- 52)

1834 – England

‘A machine which had a separate store for holding numbers (memory) and a ‘mill’ for working on them (arithmetic unit). The machine used a punched card system for specifying the sequence of instructions (input) and for obtaining results (output)’

Photograph of Charles Babbage seated ©

BABBAGE

In 1823 the mathematician and inventor Babbage was driven to attempt to build a mechanical solution to the alternative of books of mathematical tables written by teams of number crunchers to help with complicated calculations. Due to human error they were inevitably prone to mistakes. Babbage was a champion of machines and the scientific approach – he believed that if a mechanical solution could be devised then accuracy would always be assured.

Babbage designed three difference engines – mechanical devices that would compute and print mathematical and navigational tables – but never built one.

The machines were designed for mathematical calculations only.

Each number in the difference engine was represented by a column of cogwheels, and each cogwheel was marked with digits from 0 to 9. A number was set by turning the cogwheels in the column to show the right digit on each. The working model had seven number columns, each of sixteen digit cogwheels or digits. Babbage separated out the addition process from the ‘carry over’ process. The ‘mill’ (central processing unit) performed various arithmetical operations, the ‘store’ (memory) held numbers. Results from the mill were returned to the store after processing.

He also designed an analytical engine, the first programmable computer. It was much more than a calculator, rather an all-purpose computing machine. His design envisaged ‘programs’ written using loops of punched cards inspired by the Jacquard Loom. It included a reader able to process the instructions they contained, a ‘memory’, which could store the results, ‘sequential control’, and other logical features that would become components of twentieth century computers.

Drawing said to be of Ada Lovelace ©

ADA LOVELACE

‘It has no pretensions whatever to originate anything, but it can do whatever we know how to order it to perform’ wrote Lovelace about the analytical engine. Lovelace, a mathematician, is acknowledged by many as the world’s first computer programmer. Daughter of the poet Lord Byron, Lovelace worked closely with Babbage in writing instructions for his difference and analytical engines. Her writings provide the first descriptions of programming techniques. She died of cancer aged 36.

The public perception was of a ‘white elephant’. The Prime Minister Robert Peel is said to have commented that perhaps Babbage’s machine ‘should be used to calculate the time at which it would be of any use’.

Although Babbage prepared detailed drawings for thousands of parts, only a few parts were built. His project was ahead of its time and Victorian technology could not provide the precisely machined components required.

The need to develop technology as he went along meant that progress was slow and after ten years only half the parts had been made. Human computers and printed tables remained a cheaper, more practical option. One spin-off from the project, however, was the development of the first standardized screw system by Joseph Whitworth, which revolutionised engineering.

 Photograph of the internal gears of a mechanism described as the Babbage Engine ©©

BABBAGE ENGINE

In 1991, the two-hundredth anniversary of Babbage’s birth, Doran Swade and his team at the British Science museum built the difference engine number 2 (designed between 1847 and 1849). The calculation section of the engine weighs 2.6 tonnes and consists of 2400 parts.

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JANS JACOB BERZELIUS (1779-1848)

1840 – Sweden

‘An element can exist in two or more forms with different properties’

The various forms are known as allotropes. Graphite, diamond and buckyballs are three crystalline allotropes of carbon.

Berzelius contributed more than just allotropes to chemistry. When DALTON revived the idea of the atom as the unit of matter, he used circular symbols to represent atoms. Berzelius discarded Dalton’s cumbersome system and in its place introduced a rational system of chemical shorthand.

He declared ‘I shall take as the chemical sign the initial letter of the Latin name of each element. If the first two letters be common to two elements I shall use both the initial letter and the first letter they have not in common’.

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CHRISTIAN JOHANN DOPPLER (1803- 53)

1842 – Austria

‘Any source of sound or light moving away from an observer changes in frequency with reference to the observer’

photograph of a metal plaque celebrating Christan_Doppler ©

The pitch of the whistle of a train is higher when the train is approaching an observer standing on a platform and lower when it is moving away from the observer.

Doppler explained the effect by pointing out that when the source of sound is moving toward the observer, sound waves reach the ear at shorter intervals, hence the higher pitch. When the source is moving away the waves reach the ear at longer intervals, hence the lower pitch. The Doppler effect also occurs when the source of sound is stationary and the observer is moving.

Doppler predicted that a similar effect would apply to light waves.

diagram demonstrating the Doppler effect

Different colours are the optical equivalent of notes of different pitch; blue light vibrates at roughly twice the pitch of red light.

In 1929 EDWIN HUBBLE suggested that the Doppler effect applied to light coming from distant stars gives a measure of the distance and speed of distant galaxies.

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