Curiosity about the fundamental nature of matter was matched by equally
avid curiosity about the fundamental nature of electricity. Before 1850
much had been learned about the behavior of electric charge and electric
currents flowing through solids and liquids. Real progress in
understanding electric charge, however, had to wait for the development
of highly efficient vacuum pumps.
About 1854 Heinrich Geissler, a German glassblower, developed an
improved suction pump, and also succeeded in sealing into a glass tube
two wires attached to metal electrodes inside the tube. Experimenters
were then able to study the flow of electricity through a near-vacuum. A
Geissler tube is diagramed in Figure 1.
By the 1890s it had become clear that the flow of electricity through a
highly evacuated tube consisted of a negative electric charge moving at
a very high speed along straight lines between sealed-in electrodes.
Since it originated at the negative electrode, or cathode, the invisible
stream of charge was named “cathode rays.”
[Illustration: Figure 1 _Geissler Tube._]
CURRENT SOURCE
CATHODE (-)
STREAM OF ELECTRONS
VACUUM PUMP
ANODE (+)
Although many investigators contributed to knowledge about cathode rays,
the experiments of Joseph J. Thomson, a British physicist, are generally
considered to have been the most enlightening. Thomson arranged a
cathode-ray tube so that the rays could be deflected by magnets and by
electrically charged metal plates. By applying certain well-known
principles of physics, he was able to confirm an impression already held
by physical chemists, namely, that electric charge, like matter, was
“atomized”—the stream of charge consisted of a swarm of very small
particles, all alike. He succeeded also in determining that the speed of
the particles was about one-tenth the speed of light.
Probably Thomson’s most significant result was determining the ratio of
the charge of each little particle to its weight. He was able to do this
by measuring the magnetic force required to divert a stream of charged
particles. (You can do this experiment yourself with relatively simple
equipment.) This charge-to-weight ratio proved to be nearly 2000 times
greater than the already known charge-to-weight ratio for a positively
charged hydrogen atom, or ion, which until then was thought to be the
lightest constituent of matter. It remained to be determined whether
charge or weight caused the difference. Further experimentation showed
that the charges were approximately the same amount in the two cases. It
was therefore proven that the weight of the hydrogen atom, lightest of
all the atoms, was nearly 2000 times as great as the weight of one of
the little negative particles.
The name “electron” was given to the small negative particles identified
by Thomson. Since the electrons had come from the cathode, it was
apparent that the atoms in the cathode must contain electrons. Thomson
reasoned that electric current in a wire is a stream of electrons
passing successively from atom to atom and that the difference between
an electrically charged atom and a neutral atom is that the charged one
has gained or lost one or more electrons.