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C04_013A
Magnet
S
N
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a
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Electrically
charged plates
b
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Sir William Crookes
While working in a darkened laboratory, English physicist Sir
William Crookes noticed a flash of light within one of the cathode-ray tubes. A green
flash was produced by some form of radiation striking a zinc-sulfide coating that had
been applied to the end of the tube. Further work showed that there was a ray
(radiation) going through the tube. This ray, originating from the cathode and traveling
to the anode, was called a
cathode ray.
The accidental discovery of the cathode ray led
to the invention of television, which is nothing more than a cathode-ray tube. Scientists
continued their research using cathode-ray tubes, and they were fairly convinced by the
end of the 1800s that cathode rays were a stream of charged particles and that these
particles carried a negative charge. The exact value of the negative charge was not yet
known.
Because changing the metal that makes up the electrodes or varying the gas (at very low
pressure) in the cathode-ray tube did not affect the cathode ray produced, researchers
concluded that the ray’s negative particles were found in all forms of matter. These
negatively charged particles that are part of all forms of matter are now known as
electrons.
Some of the experiments used to determine the properties of the cathode ray
are shown in
Figure 7
.
Mass and charge of the electron
In spite of the progress made from all of the cathode-ray tube experiments, no one
succeeded in determining the mass of a single cathode-ray particle. Unable to measure
the particle’s mass directly, English physicist J. J. Thomson (1856–1940) began a series
of cathode-ray tube experiments at Cambridge University in the late 1890s to determine
the ratio of its charge to its mass.
Charge-to-mass ratio
By carefully measuring the effects of both magnetic and
electric fields on a cathode ray, Thomson was able to determine the charge-to-mass
ratio of the charged particle. He then compared that ratio to other known ratios.
Figure 7
A tiny hole located at the center of the anode produces a thin beam of electrons. A phosphor
coating allows the position of the beam to be determined as it strikes the end of the tube.
Because the cathode ray is deflected in a magnetic field,
the particles in the ray must be charged.
Because the cathode ray is deflected toward the positively
charged plate by an electric field, the particles in the ray
must have a negative charge.
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Module 3 • The Structure of the Atom