1895 – Germany

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‘X-Rays are high energy radiation given off when fast-moving electrons lose energy very rapidly’

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1897 – England

’Not only was matter composed of particles not visible even with the modern microscope, as scientists from DEMOCRITUS to DALTON had surmised, but those particles were themselves composed of even smaller components’

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By the end of the nineteenth century scientists had cleared up much of the confusion surrounding atomic theory. The discovery of the sub-atomic particle was made in April 1897. They believed that they now largely understood the properties and sizes of the atoms of elements; without question, hydrogen was the smallest of all.

When JJ Thomson announced the discovery of a particle one thousandth the mass of the hydrogen atom the particles were named ‘electrons’ and have been a fundamental part of the understanding of atomic science ever since.

Thomson was investigating the properties of cathode rays, now known to be a simple stream of electrons, but at the time the cause of widespread debate. The rays were known to be visible, like normal light, but they were quite clearly not normal light. He devised a series of experiments, which would apply measurements to the cathode rays and clarify their nature. The rays were created by passing an electric charge through an airless or gasless discharge tube.

By improving the vacuüm in the tube, it was demonstrated that the rays could be deflected by electric and magnetic fields. Thomson drilled a hole in the anode of the tube to allow the mysterious rays from the cathode to pass through. In the space after the anode, he arranged that a magnetic force field from a magnet would tug the cathode rays in one direction, and an electric force field between two electrically charged metal plates would tug them in the opposite direction. The rays would eventually strike the glass wall of the tube to create a familiar greenish spot of light on the phosphor-coated tube.

Thomson concluded that the rays were made up of particles, not waves. He saw that the properties of the particles were negative in charge and didn’t seem to be specific to any one element; they were the same regardless of the gas used to transport the electric discharge, or the metal used at the cathode. From his findings he concluded that cathode rays were made up of a jet of ‘corpuscles’ and, more importantly, that these corpuscles were present in all elements. Thomson devised a method of measuring the mass of the particles and found them to be a fraction of the weight of the hydrogen atom.

The position of the spot indicated how much the beam of cathode rays had been deflected. The deflection could be made zero by adjusting the magnetic and electric forces so that they perfectly balanced. In such a situation, Thomson could read off the strength of the electric force. He knew in theory how the magnetic force on a charged particle depends on its speed. By equating the two forces, he was able to deduce the speed of the cathode rays. The deflection was also influenced by the electric charge carried by the cathode ray particles, and their mass. The larger the charge, the greater the force the particles felt and the greater their deflection, the smaller the mass, the easier it was for any force to push the particles about and again, the greater their deflection.

Independent evidence from electrolysis (passing electricity through liquids) that electric charge came in discreet chunks, which he assumed to be carried by individual cathode ray particles, enabled Thomson to calculate their mass.
He arrived at a figure that was a thousand billion billion billionth of a kilogram – a 1000th of the mass of a hydrogen atom.

Knowing the deflection of the dot and the velocity of the particles (the slower the particles, the longer they were exposed to the electric force and the greater the deflection of the glowing dot), Thomson expected to be able to deduce their charge and mass. What he actually deduced was a combination of their charge and mass.

Atoms were made of smaller things, but the fundamental building-blocks were not hydrogen atoms, as had been maintained by PROUT.

Thomson’s particles were christened ‘electrons’ and were the first subatomic entities. Thomson visualized a multitude of tiny electrons embedded ‘like raisins in a plum pudding’ in a diffuse ball of positive charge.

‘The atom is a sphere of positively charged protons in which negatively charged electrons are embedded in just sufficient quantity to neutralise the positive charge’

This was the accepted picture of the atom at the start of the twentieth century until RUTHERFORD found a way to probe inside the atom in 1911.

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1898 – France



‘1903 – Awarded the Nobel-Prize for Physics jointly with Marie and Pierre Curie’

picture of a rock displaying fluorescence under short wavelength radiation

The phenomenon of fluorescence – displayed under short wavelength radiation

Stimulated by WILHELM CONRAD ROENTGEN’s discovery of X-rays in 1895, Becquerel chanced upon the phenomenon that is now known as radioactivity in 1896. The Frenchman believed that Röntgen’s X-rays were responsible for the fluorescence displayed by some substances after being placed in sunlight. Although he was wrong to assume that fluorescence had anything to do with X-rays, he tested large numbers of fluorescent minerals.

He found that uranium, the heaviest element, caused an impression on a covered photographic plate, even after being kept in the dark for several days, and concluded that a phenomenon independent of sunlight induced luminescence.
Investigation isolated the uranium as the source of ‘radioactivity’, a name given to the occurrence by Mme. Curie.

The SI unit of radioactivity, the becquerel is named in his honour.

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NEXT button - MARIE CURIE (1867-1934) PIERRE CURIE (1859-1906)TIMELINE

  • Radioactivity (aps.org)
  • MARIE CURIE (1867-1934) PIERRE CURIE (1859-1906)

    1898-1902 – France

    ‘Pitchblende, the ore from which uranium is extracted, is much more radioactive than pure uranium. The ore must therefore contain unknown radioactive elements’

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    Photograph of pierre_curie ©


    Following the discovery of radioactivity by HENRI BECQUEREL (1852-1908) in 1896, Marie Curie conclusively proved that radioactivity is an intrinsic property of the element in question and is not a condition caused by outside factors.

    She correctly concluded that pitchblende contained other, more radioactive elements than uranium.
    The Curies isolated two new radioactive elements, polonium and radium, from pitchblende. The discovery of new elements by their radioactivity was proof that radioactivity was a property of atoms.

    image of two pages from MarieCurie's notebook, which remains radioactive

    Even today, Marie Curie’s notebooks of her studies remain too radioactive to handle.

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    NEXT buttonTHE ATOM


    MAX PLANCK (1856-1947)

    1900 – Germany

    ‘Energy is not a continuous quantity but it is quantised; it flows in discrete packets or quanta. When particles emit energy they do so only in quanta’

    According to Quantum theory, the energy (E) of one quantum (photon) is given by E = hf where f is the frequency of radiation and h is Planck’s constant.
    Its value is 6.63 x 10-34 joules per second

    h is a tiny number, close to zero, but it is has a finite value. This implies energy is released in discrete chunks, a revolutionary notion.

    photo portrait of MAX PLANCK ©


    By the late 1800s the science of thermodynamics was developing to the point that people were beginning to understand the nature of energy.
    The traditional view was that energy was released in a continuous stream and that any amount of energy could be indefinitely divided into smaller and smaller ‘lumps’. Planck’s work on the laws of thermodynamics and black body radiation led him to abandon this classical notion of the dynamic principles of energy and formulate the quantum theory, which assumes that energy changes take place in distinct packages, or quanta, that cannot be subdivided. This successfully accounted for certain phenomena that Newtonian theory could not explain.

    The basic laws of thermodynamics recognised that energy could not be created or destroyed, but was always conserved. The second law was drawn from an understanding that heat would not pass from a colder body to a hotter body.
    The study of thermodynamics was based on the assumption that matter was ultimately composed of particles. LUDWIG BOLTZMAN had proposed an explanation of thermodynamics, saying the energy contained in a system is the collective result of the movements of many tiny particles rattling around. He believed the second law was only valid in a statistical sense; it only worked if you added up all the bits of energy in all the little particles.
    Among his detractors was Max Karl Ernst Ludwig Planck.

    Planck began his work on the second law of thermodynamics and the concept of entropy. He investigated how materials transform between solid, liquid and gaseous states. In doing so he found explanations for the laws governing the differing freezing and boiling points of various substances.
    He also looked at the conduction of electricity through liquid solutions (electrolysis).

    In the mid 1890s Planck turned his attention to the question of how heated substances radiate energy. Physicists were aware that all bodies radiate heat at all frequencies – although maximum radiation is emitted only at a certain frequency, which depends on the temperature of the body. The hotter the body, the higher the frequency for maximum radiation. (Frequency is the rate per second of a wave of any form of radiation).

    Planck had been considering formulae for the radiation released by a body at high temperature. Using ideas developed by ROBERT KIRCHOFF, he knew it should be expressible as a combination of wavelength frequency and temperature. For a theoretical ‘black body’, physicists could not predict expressions that were in line with the behaviour of hot bodies at high frequencies and were in agreement with other equations showing their nature at low frequencies. Thus no law could be found which fitted all frequencies and obeyed the laws of classical physics simultaneously.
    Plank resolved to find a theoretical formula that would work mathematically, even if it did not reflect known physical laws. His first attempts were partially successful, but did not take into account any notion of particles or quanta of energy, as he was certain of the continuous nature of energy. In an ‘act of despair’ he renounced classical physics and embraced quanta.

    The final straw had been a concept developed by John Rayleigh and James Jeans that became known as the ‘ultraviolet catastrophe’ theory. They had developed a formula that predicted values for radiation distribution and worked at low frequencies, but not at high frequencies. It was at odds with Planck’s formula, which worked for high frequencies but broke down at low frequencies. In June 1900 Rayleigh had pointed out that classical mechanics, when applied to the oscillators of a black-body, leads to an energy distribution that increases in proportion to the square of the frequency. This conflicted with all known data.

    Planck’s answer was to introduce what he called ‘energy elements’ or quanta and to express the energy emitted as a straightforward multiplication of frequency by a constant, which became known as ‘Planck’s constant’ (6.6256 x 10-34 Jsec-1). This only works with whole number multiples which means for the formula to have any practical use one must accept the radical theory that energy is only released in distinct, non-divisible chunks, known as ‘quanta’, or for a single chunk of energy, a ‘quantum’. This completely contradicts classical physics, which assumed that energy is emitted in a continuous stream. The individual quanta of energy were so small that when emitted at the everyday large levels observed, it appears that energy could seem to be flowing in a continuous stream.
    Thus classical physics was cast into doubt and quantum theory was born.

    Planck announced his theory on December 14 1900 in his paper ‘On the Theory of the Law of Energy Distribution in the Continuous Spectrum’. Planck said ‘energy is made up of a completely determinate number of finite equal parts, and for this purpose I use the constant of nature h = 6.55 x 10-27(erg sec)’

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    When ALBERT EINSTEIN was able to explain the ‘photoelectric’ effect in 1905, suggesting that light is emitted in quanta called ‘photons’, by applying Planck’s theory – and likewise NIELS BOHR in his explanation of atomic theory in 1913 – the abstract idea was shown to explain physical phenomena.

    Planck was awarded the Nobel Prize for Physics in 1918.

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    NEXT buttonHEAT


    ALFRED NOBEL (1833- 96)

    1901 – Sweden

    • 1866 – Invents dynamite

    • 1876 – Invents blasting gelatin

    • 1886 – Invents ballistite

    • 1896 – Nobel Foundation set-up to comply with the terms of Nobel’s will

    • 1901 – First Nobel Prizes awarded

    A Swede educated in Russia, France and the United States, Alfred Nobel was a chemist who set up a factory to manufacture the relatively unstable nitro-glycerine to serve the civil-engineering market. After a disaster in 1864, Nobel found a way to stabilise the liquid explosive with kieselguhr, which he called dynamite. He went on to develop blasting gelatin, ballistite and a series of detonators.

    The success of Nobel’s dynamites was compounded by his holdings in oil, leading to vast personal wealth. He left much of his fortune to funding the establishment of a series of awards, one of which included an accolade for peace.
    There was another dedicated to literature, with the remaining three presented for achievements in the sciences. The first Nobel prizes for medicine (or physiology), physics and chemistry were awarded in 1901.

    The prizes are awarded annually, according to the terms of Nobel’s will; ‘to those who, during the preceding year, shall have conferred the greatest benefit on mankind.’

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    ALBERT EINSTEIN (1879-1955)

    1905 – Switzerland

    1. ‘the relativity principle: All laws of science are the same in all frames of reference.
    2. constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
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    The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.

    Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
    The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.

    ‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’

    This equation shows that mass and energy are mutually convertible under certain conditions.

    The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.

    Two of his early papers described Brownian motion and the ‘photoelectric’ effect (employing PLANCK’s quantum theory and helping to confirm Planck’s ideas in the process).

    1915 – Germany

    ‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’

    From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.

    Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.

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