J. j thompson (1856 – 1940)



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J.J Thompson (1856 – 1940)

In 1897 the physicist Joseph John (J. J.) Thomson discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube*—an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge to mass ratio.

* These were called "Crookes Tubes" named after the original designer. They would later be called "Cathode Ray Tubes". Experiments were performed on these tubes in which high voltage electrical current was passed between the two electrodes. Ray like emanations proceeded from the Cathode electrode to the Anode electrode. Since these emanations originated from the Cathode electrode they would be called "Cathode Rays". J.J. Thompson designed some special tubes that investigated the properties of these "Cathode Rays". He designed a tube that allowed a beam of these Cathode Rays to impact against the surface of a Zinc Sulfide coated screen. As the rays impacted on the surface, it emitted a spark of light so that the invisible ray's path could be observed. He then proceeded to bring an electrical field consisting of a positive plate and a negative plate near the vacinity of the Rays. When the electrical current of the electrical field was turned on, the path of the "rays" was deflected away from the negative plate and toward the positive plate. This was a clear indication that the so called rays possessed a negative charge. Another Crookes tube design had an object, a Maltese Cross, placed just past the exit path of the cathode rays as they went from the cathode to the anode.

A shadow of the cross was cast upon the front of the tube. The only way that the "rays" could cast a shadow impression on the back of the tube was if they went past the exit path and struck the cross. This would strongly indicate that the rays possessed momentum, but in order for anything to have momentum that would mean that the rays would have to possess mass since momentum = mass x velocity.

But if the "rays" possessed mass that would mean that they were not rays (pure radiation) at all but particles with a finite mass!!

Later he estimated the value of the charge itself. In 1904 he suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved.



Ernest Rutherford (1871 – 1937)

New Zealander-English physicist who was born in Nelson, New Zealand, attended school in Nelson and Marlborough, and finished his tertiary education in Canterbury, New Zealand before traveling to England. Rutherford is best known for devising the names alpha, beta, and gamma rays to classify various forms of "rays" which were poorly understood at his time (alpha and beta rays are particle beams, while gamma rays are a form of high-energy electromagnetic radiation ).

Rutherford deflected alpha rays with both electric and magnetic fields in 1903. He also observed that the intensity of radioactivity fell off with time, and named the halving time the "half-life. " In 1906, his students Geiger and Marsden conducted the classic gold foil alpha particle scattering experiment which showed large deflections for a small fraction of incident particles. This led Rutherford to propose that the atom was "nuclear." For his discoveries, Rutherford was awarded the 1908 Nobel Prize in chemistry. He much resented that the prize was in chemistry rather than physics, and his acceptance speech made a remark to the effect that he had seen many transformations in his studies, but never one more rapid than his own from physicist to chemist.

Rutherford suggested that the simplest possible rays must be those obtained by hydrogen and that these must be the fundamental positively charged particle, which he dubbed the proton in 1914. In 1917, he passed alpha particles through a gas of nitrogen and occasionally observed scintillation of hydrogen impacting on his screen. He concluded that the alpha particles were knocking protons out of the nitrogen atoms, and thus that he had made the first observation of nuclear reactions.

Rutherford's image appears on New Zealand's $100 note, that country's largest denomination of paper currency.

One particularly memorable quote attributed to Rutherford is "All science is either physics or stamp collecting" (Birks 1963).



James Chadwick (1891-1974)

English physicist who worked with Rutherford on the bombardment of elements with alpha particles. In the 1920s, Rutherford and Chadwick attempted to find a uncharged elementary particle, but failed. Then, Chadwick repeated the earlier experiment of Bothe and Frédéric and Iréne Joliot-Curie by exposing beryllium to alpha particles. The Be then gave off radiation which could eject protons from the paraffin.



In 1932, Chadwick showed that a neutral particle beam was the only way to interpret the ejection of protons. This amounted to the discovery of the neutron. For this, he received the 1935 Nobel Prize in physics. With Goldhaber, Chadwick also determined the structure of the deuteron. Furthermore, he showed that the spectrum of beta particles emitted by radioactive elements was continuous, providing the experimental need for neutrinos, and discovered deviation from the r-2 law for deep Rutherford scattering.

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