J.J. Thomson

Sir Joseph John "J. J." ThomsonBorn: 18 December 1856

Died: 30 August 1940Born in Cheetham Hill, Manchester, England

• Discovered the electron and isotopes in 1897 - He made cathode rays by firing electrical currents through glass pipes filled with low-density gas. Thomson measured the ratio of the mass of the cathode ray to its electrical charge. What he found was that the ratio was always the same, regardless of what elemental gas was in the pipe. These results indicated that the current inside the cathode ray tube was made of tiny particles that carried a negative charge -- he named these particles electrons. Since the mass to charge ratio was the same no matter what gas he used inside the glass tube, Thomson reasoned that electrons must be common to all atoms, and that all electrons must be the same.

Sir William Crookes

Sir William CrookesBorn: 17 June 1832Died: 4 April 1919

Born in London

• Discovered cathode ray tube in 1879 proving cathode rays real

- Discovered cathode rays travel in straight lines from the cathode, cause glass to fluoresce, impart a negative charge to objects they strike, are deflected by electric fields and magnets to suggest a negative charge, cause pinwheels in their path to spin indicating they have mass.

- he created a bent Geissler tube in 1875.He noticed that the glow was the most intense opposite the negative electrode, also called the cathode. Crookes reasoned that rays traveled from the cathode and then hit the end of the tube. Because of this, Crookes named these rays cathode rays.

John Dalton

John DaltonBorn: 6 September 1766

Died: 27 July 1844Born in Eaglesfield, Cumbria

• Proposed the Atomic theoryFive main points of Dalton's atomic theory• Elements are made of extremely small

particles called atoms.• Atoms of a given element are identical in

size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

• Atoms cannot be subdivided, created, or destroyed.

• Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

• In chemical reactions, atoms are combined, separated, or rearranged.

Albert Einstein

Albert EinsteinBorn: 14 March 1879Died: 18 April 1955

Born in Ulm, Kingdom of Württemberg

• In a 1905 paper, Einstein postulated that light itself consists of localized particles. Einstein's light quanta were nearly universally rejected by all physicists, including Max Planck and Niels Bohr. This idea only became universally accepted in 1919, with Robert Millikan's detailed experiments on the photoelectric effect, and with the measurement of Compton scattering.

Eugen Goldstein

Eugen GoldsteinBorn: September 5, 1850Died: December 25, 1930

Born in Gleiwitz, Upper Silesia

• Discovered the electron• In the 1870s Goldstein undertook his own investigations of

discharge tubes, and named the light emissions studied by others kathodenstrahlen, or cathode rays. He discovered several important properties of cathode rays, which contributed to their later identification as the first subatomic particle, the electron. He found that cathode rays were emitted perpendicularly from a metal surface, and carried energy. He attempted to measure their velocity by the Doppler shift of spectral lines in the glow emitted by Crookes tubes.

• In 1886, he discovered that tubes with a perforated cathode also emit a glow at the cathode end. Goldstein concluded that in addition to the already-known cathode rays, later recognized as electrons moving from the negatively-charged cathode toward the positively-charged anode, there is another ray that travels in the opposite direction. Because these latter rays passed through the holes, or channels, in the cathode, Goldstein called them Kanalstrahlen, or canal rays. They are composed of positive ions whose identity depends on the residual gas inside the tube. It was another of Helmholtz's students, Wilhelm Wien, who later conducted extensive studies of canal rays, and in time this work would become part of the basis for mass spectrometry.

Ernest Rutherford

Ernest RutherfordBorn: 30 August 1871Died: 19 October 1937

Born in Spring Grove (now Brightwater), near Nelson, New Zealand

• Essentially discovered protons and neutrons• Ernest Rutherford publishes his atomic theory

describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide. Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.

Robert Millikan

Robert Andrews MillikanBorn: March 22, 1868

Died: December 19, 1953Born in Morrison, Illinois

• Proved Einstein’s photon theory whilst trying to disprove it• When Einstein published his seminal 1905 paper on the

particle theory of light, Millikan was convinced that it had to be wrong, because of the vast body of evidence that had already shown that light was a wave. He undertook a decade-long experimental program to test Einstein's theory, which required building what he described as "a machine shop in vacuo" in order to prepare the very clean metal surface of the photo electrode. His results confirmed Einstein's predictions in every detail, but Millikan was not convinced of Einstein's interpretation, and as late as 1916 he wrote, "Einstein's photoelectric equation... cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation," even though "it actually represents very accurately the behavior" of the photoelectric effect. In his 1950 autobiography, however, he simply declared that his work "scarcely permits of any other interpretation than that which Einstein had originally suggested, namely that of the semi-corpuscular or photon theory of light itself.”