Sir Joseph John Thompson

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Sir JosephJohn Thompson
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Introduction

• J.J. Thompson (born 1856) was a Mathematical
  Physicist
• Most notable for the discovery of the electron as
  an atomic particle
• He attended Owens College, Manchester, before
  moving to Trinity College, Cambridge, where he
  stayed until his death in 1940

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The Electron
Excuse me... how can you discover a particle so
small that nobody has ever seen one?

• In 1897, J.J. Thompson, along with a group of his
  graduate students, set out to investigate the a
  long-standing puzzle known as cathode rays
• He speculated that these mysterious rays are
  streams of particles much smaller than atoms,
  that they are in fact minuscule pieces of atoms
• He suggested that these corpuscles (as he named
  them) might make up all of the matter in atoms
• Most people at the time thought that the atom was
  indivisible, the most fundamental unit of matter

J.J. Thomson experimenting, in the Cavendish Lab
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Cathode Ray Tube
Could anything at first sight seem more
impractical than a body which is so small that
its mass is an insignificant fraction of the mass
of an atom of hydrogen?

• Thompson 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 interpreted the deflection of the rays by
  electrically charged plates and magnets as
  evidence of bodies much smaller than atoms

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Other works

• Thompson managed to estimated the charge to mass
  ratio of an electron.
• In 1904 he suggested a model of the atom as a
  sphere of positive matter in which electrons are
  positioned by electrostatic forces.
• The techniques he used in his last important
  experiment led to the development of the mass
  spectroscope (now the mass spectrometer) by his
  assistant Francis Aston

Early mass spectrometer
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Notable achievements