Joseph John Thomson

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Joseph John Thomson
Born December 18th 1856, near Manchester, England
Attended Owens College in Manchester, where his
professor of Mathematics encouraged him to apply
for a scholarship at Trinity College.
He won the scholarship and in 1880 he finished
second in his class behind Joseph Larmor.
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Do atoms have parts?
J.J. Thomson suggested that they do. He advanced
the idea that cathode are really streams of very
small pieces of atoms. Three experiments led him
to this.
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First Experiment

• He built a cathode ray tube ending in a pair of
  metal cylinders with a slit in them, which were
  connected to an electrometer which catches and
  measures electric charge

• He wanted to see if by bending the rays with a
  magnet he could separate the charge from the rays.

• He found that he could not do this.

Cathode rays pass from the tube in the upper left
into the larger bulb, where they are deflected
with a magnetic field. When they are bent so as
to enter the slits in the cylinders, the
electrometer measures the charge transferred to
the cylinder.
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Second Experiment

• He tried a new approach, by extracting all the
  gas from a tube.

• He found that he could now separate the charge
  from the rays

Rays from the cathode (C) pass through a slit in
the anode (A) and through a slit in a grounded
metal plug (B). An electrical voltage is
established between aluminium plates (D and E),
and a scale pasted on the outside of the end of
the tube measures the deflection of the rays.
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In Conclusion

• From these experiments he found that cathode rays
  are charges of negative electricity carried by
  particles of matter but he didnt know what these
  particles were.

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Third Experiment

• He sought to determine the basic properties of
  the particles.

• He could measure how much the rays were bent by a
  magnetic field and how much energy they carried,
  from this he could calculate the ratio of the
  mass of a particle to its electric charge.

• The results showed that the mass to charge ratio
  for cathode rays turned out to be over one
  thousand times smaller than that of a charged
  hydrogen atom.

• Either the cathode rays carried and enormous
  charge or else they were amazingly light relative
  to their charge.

Rays originate at the cathode on the left and
pass through a slit in the anode into a bell jar
containing gas at low pressure. The deflected
paths of the rays are photographed against a
ruled glass plate.
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Thomson announced
We have in the cathode rays matter in a new
state, a state in which the subdivision if matter
is carried very much further than in the ordinary
gaseous state a state in which all matter is of
one and the same kind this matter being the
substance from which all the chemical elements
are built up.
He had found something smaller than an atom,
which he knew was negatively charged, and that
made up all chemical elements.