As early as 1886 Eugen Goldstein, who was working with cathode rays,
also studied rays that moved in the opposite direction. Since the
cathode rays (electrons) were negatively charged, rays moving in the
opposite direction would have to be positively charged. In 1907 J. J.
Thomson called them “positive rays”.
Once Rutherford worked out the nuclear structure of the atom, it seemed
clear that the positive rays were atomic nuclei from which a number of
electrons had been knocked away. These nuclei came in different sizes.
Were the nuclei single particles—a different one for every isotope of
every element? Or were they all built up out of numbers of still smaller
particles of a very limited number of varieties? Might it be that the
nuclei owed their positive electrical charge to the fact that they
contained particles just like the electron, but ones that carried a
positive charge rather than a negative one?
All attempts to discover this “positive electron” in the nuclei failed,
however. The smallest nucleus found was that produced by knocking the
single electron off a hydrogen atom in one way or another. This hydrogen
nucleus had a single positive charge, one that was exactly equal in size
to the negative charge on the electron. The hydrogen nucleus, however,
was much more massive than an electron. The hydrogen nucleus with its
single positive charge was approximately 1837 times as massive as the
electron with its single negative charge.
Was it possible to knock the positive charge loose from the mass of the
hydrogen nucleus? Nothing physicists did could manage to do that. In
1914 Rutherford decided the attempt should be given up. He suggested
that the hydrogen nucleus, for all its high mass, should be considered
the unit of positive electrical charge, just as the electron was the
unit of negative electrical charge. He called the hydrogen nucleus a
“proton” from the Greek word for “first” because it was the nucleus of
the first element.
[Illustration: _One proton balances 1837 electrons._]
Why the proton should be so much more massive than the electron is still
one of the unanswered mysteries of physics.