Joseph John Thomson was born on December 18, 1856 in Manchester.
His father died when he was only 16. Young Thomson attended Owens
College in Manchester, where his Professor of Mathematics encouraged
him to apply for a scholarship at Trinity College, one of the most
prestigious of the colleges at Cambridge University. Thomson finished
second in his class in the graduation examination in mathematics
in 1880 and won the scholarship. Trinity College gave him a fellowship
where he stayed upon. He engaged himself in developing mathematical
models that would reveal the nature of atoms and electromagnetic
The Cavendish Laboratory at Cambridge was founded
in 1871 with James Clerk Maxwell - who developed the basic equations
of electromagnetism as the first Cavendish Professor. At 28, the
young Thomson was chosen to be the third Cavendish Professor in
1884 following Maxwell and Lord Rayleigh. True, he was inexperienced
in doing experiments, but he learned quickly. Supported by his administration
and teaching, many important experiments on electromagnetism and
atomic particles were performed. Many outstanding Physicists received
their early training at the Cavendish, including 7 Nobel Prize winners
and 27 Fellows of the Royal Society. Thomson took a keen interest
in the work of all the young researchers, daily checking on their
progress and making suggestions for improvement.
J.J. Thomson married Rose Paget on January 22,
1890. She was among the researchers at the Cavendish as one of the
first generation of women permitted into advanced university studies.
She performed experiments of soap films in 1889 after attending
some of Thomson's lectures. They had two children: George Paget
Thomson who flourished into a prominent Physicist himself and won
Nobel Prize for discovery of the diffraction of electron by crystals
in 1937. Their daughter John Paget Thomson often accompanied his
father in his travels.
In the paper published in Philosophical Magazine
100 years ago in October 1997, Thomson reported that cathode rays
were charged particles, which he called "corpuscles".
It is hard to recall any discovery since then that has had
more impact on not only physics but science, technology and our
daily lives. We shall briefly follow the course of history
in this article that made it possible.
Indeed, as far back as 1705, it had been noticed
that sparks from an electrical machine would jump further in rarified
air than in air at normal pressures. Watson in 1748 observed an
aurora borealis like "arch of lambent (i.e. glowing) flame"
in a glass tube of rarified air 32 inches long. In 1838 Faraday
sent a current from an electrostatic machine through a glass tube
containing air at low pressure and observed a purple glow extending
from the positive electrode, or anode, at one end, almost to the
negative electrode, or cathode, at the other. The cathode was covered
with a glow, and there was a dark space between this glow and the
purple column. The dark space has since been called the "Faraday
Dark Space." (The colour of the internal glow of such tubes
depends on the kind of gas present in the tube. Neon, at pressures
approximately one hundredth that of atmospheric pressure, glows
with a bright orange colour when current passes through it; helium,
a pinkish white; mercury vapour, a light greenish blue.)
Although Faraday observed a number of interesting
phenomena, he was limited by the fact that the suction pumps available
at the time for reducing the gas pressure were not too efficient.
A great step forward was made, about 1854, when H.Geissler, a German
glass blower of exceptional skill, not only developed an improved
vacuum suction pump, but succeeded in sealing into glass tubes wires
attached to metal electrodes. The evacuated Geissler tubes which
he made were particularly suitable for the study of the passage
of electricity through gases at low pressure, and with them J. Plucker,
in Germany, made numerous experiments between the years 1858 and
1862. Among other things, he observed that the tube in the vicinity
of the cathode, i.e. the electrode attached to the negative side
of the source of potential, emitted a green glow or luminescence.
The position of the glow could be changed by bringing a magnet up
to the tube.
The studies of electrical discharge through gases
were continued in Germany by Plucker's pupil, W.Hittorf (1869),
and by E.Goldstein (1876). From their observations they concluded
that the luminescent glow on the tube was caused by "rays"
originating at the cathode, which Goldstein consequently called
cathode rays. The rays could be deflected by a magnet and were also
able to cast a shadow of an obstacle placed in their path, showing
that they traveled in straight lines.
Between the years 1879 and 1885 the English scientist
William Crookes, who designed improved vacuum dischage tubes, made
a very comprehensive series of investigations of the electrical
discharge. From these he concluded that the cathode rays actually
consisted of a stream of negatively charged particles, which were
expelled from the cathode _ the negative electrode _ with extremely
high velocities. This view of the nature of cathode rays supported
a suggestion made in 1872 by C.F.Varley, but it was opposed by many
European physicists, including such eminent men as E.Wiedemann (1880),
H.Hertz (1883) the discoverer of radio waves, and (1894) P. Lenard.
The latter group thought the cathode rays were an electromagnetic
wave motion or vibration, analogous to light waves but of shorter
wave length. If the rays are really a stream of charged particles
then they should be deflected by passage through an electric field,
as well as by a magnetic field, although Goldstein, in spite of
several trials, had failed to observe any such effect. But the deflection
of cathode rays in the field of magnet was an accepted fact, and
this could not be explained if the rays were similar to light waves.
In an apparently decisive experiment, performed
by J. Perrin in France in 1895, the cathode rays were allowed to
fall on a device known as a Faraday cylinder, connected to an electrometer
by means of which the sign and magnitude of electric charge could
be determined. It was found that a negative charge collected in
the cylinder, and so it was argued that the rays were made up of
negative particles. Objection was taken to this conclusion on the
grounds that negatively charged particles might well be ejected
from the cathode, but there was no proof that they are identical
with the cathode rays.
On the basis of his experiments, J.J. Thomson proposed
a model of internal atomic structure according to which
atoms consisted of a positively charged substance (positive
electric fluid) distributed uniformly over the entire body
of the atom, with negative electrons embedded in this continuous
positive charge like seeds in a watermelon, or raisins in
pudding. Since electrons repel each other but are, on the
other hand attracted to the centre of the positive charge,
they were supposed to assume certain stable positions inside
the body of the atom. Ernest Rutherford (1871-1937) and
Hans Geiger together bombard d tbin pieces of gold with
alpha particles. Most of the alpha particles passed right
through the foil, and the result was exactly what the experimenters
expected based on Thomson's model of the atom. But some
of alpha particles struck the gold foil and were deflected
at a sharp angle often 90ø or more (J J Thomson's
Model). This amazed Rutherford, who remarked "It was
as though you have fired a 15-inch shell at a piece of tissue
paper and it came back and hit you". Early in 1911
Rutherford exclaimed to Geiger, "I know what the atoms
looks like!". Rutherford put together a new idea of
the atom: what if all the positively charged particles in
the atom were not spread like a fluid throughout the atom
as Thomson had thought but were lumped together in the centre
in one tiny area, or "nucleus"? Most of the atom's
mass would be contained in the nucleus, and an equal number
of negatively charged electrons would be found in motion
somewhere outside the nucleus. Undoubtedly, it was a compelling
idea - a sort of tiny planetary system that resembled the
larger solar system we are living in.
The required proof was provided in 1897 by J.J.
Thomson (Fig. 1), the famous English physicist, whose work has had
a profound effect, both direct and indirect, on the study of atomic
structure. In the first place, he repeated Perrin's experiment and
confirmed that charged particles are emitted by the cathode. But,
in addition, he showed that when the cathode rays are deflected
by a magnetic field, as indicated by the change in position of the
luminescence they produce, the negatively charged particles are
correspondingly deflected. Further, Thomson succeeded, where Goldstein
and others had failed, in deflecting the path of the cathode rays
by means of an electric field. Previous failures had been due to
excessive ionization of the gas still present in the discharge tube,
thus offsetting the effect of the electric field. By working at
very low pressures Thomson minimized the influence of this ionization
and then he was able to observe the anticipated deflection. We shall
trace his efforts that established that the cathode rays are actually
a stream of particles carrying negative electrical charges. Indeed,
this was the result of a number of converging studies by several
prominent physicists, which we shall briefly consider.
First, in a variation of an 1895 experiment by
Jean Perrin, Thomson built a cathode ray tube ending in a pair of
metal cylinders with a slit in them . These cylinders were in turn
connected to an electrometer, a device for catching and measuring
electrical charge. Perrin had found that cathode rays deposited
an electric charge. Thomson wanted to see if, by bending the rays
with a magnet, he could separate the charge from the rays. He found
that when the rays entered the slit in the cylinders, the electrometer
measured a large amount of negative charge. The electrometer did
not register much electric charge if the rays were bent so they
would not enter the slit. As Thomson saw it, the negative charge
and the cathode rays must somehow be stuck together: you cannot
separate the charge from the rays.
All attempts had failed when physicists tried
to bend cathode rays with an electric field. Now Thomson thought
of a new approach. A charged particle will normally curve as it
moves through an electric field, but not if it is surrounded by
a conductor, say, a sheath of copper. Thomson suspected that the
traces of gas remaining in the tube were being turned into an electrical
conductor by the cathode rays themselves. To test this idea, he
took great pains to extract nearly all of the gas from a tube, and
found that now the cathode rays did bend in an electric field after
Thomson concluded from these two experiments,
"I can see no escape from the conclusion that [cathode rays]
are charges of negative electricity carried by particles of matter".
But, he continued, "What are these particles? are they atoms,
or molecules, or matter in a still finer state of subdivision?".
This is the famous "duck argument". If it looks like a
duck, quacks like a duck and waddles like a duck, then we have good
reason to believe it is a duck!
Thomson's third experiment sought to determine
the basic properties of the particles. Although he couldn't measure
directly the mass or the electric charge of such a particle, he
could measure how much the rays were bent by a magnetic field, and
how much energy they carried. From this data he could calculate
the ratio of the mass of a particle to its electric charge (m/e).
He collected data using a variety of tubes and using different gases.
The method essentially involved sending the beam into an electrically
shielded collector, as in the Perrin's experiment, but in this case
making the collector physically small. The beam gave up its charge
to the collector and also heated it by mechanical impact. The quantity
of heat energy, H, given to the collector in a given interval of
time T could be determined from its mass, specific heat, and temperature
rise. This charge in temperature could be measured by means of a
very light thermocouple attached to the collector. The total charge,
Q delivered to the collector could be measured by a sensitive electrometer.
Electron As A Particle
In 1899, Thomson set out to resolve the
doubt concerning the significance of the e/m values of the
"corpuscles" by determining directly their charge
as well as the charge to mass ratio. Unfortunately, this could
not be done with the cathode-ray particles, and so he turned
to another source. It was well-known towards the end of the
19th century that ultra-violet light falling on certain metals,
particularly zinc, was associated with the emission of negatively
charged particles, a phenomena known as the photoelectric
effect. Thomson determined the en ratio for these particles
by means of electric and magnetic fields and found it to be
virtually the same as for the cathode ray corpuscles. Charged
particles emitted by an incandescent filament, i.e. by the
thermionic effect, also had a similar e/m value. His estimate
of the electronic charge of the photoelectric particles turned
out to be similar to the unit electronic charge. In view of
the consistency of the e/m for the negatively charged particles
produced in different ways, it was reasonable to conclude
that the particles were identical.
In the words of Thomson: "The experiments
just described, taken in conjunction with previous ones ....
on cathode rays, show that in gases at low pressures negative
electrification, though it may be produced by very different
means, is made up of units each having a charge of electricity
of a definite size; the magnitude of this negative charge
is .... equal to the positive charge carried by the hydrogen
atom (ion) in the electrolysis of solutions".
Because the charge on the particles present
in the cathode rays, and associated with the thermionic and
photoelectric effect, was identical with the elementary electric
charge, the name electron originally intended by G. Johnstone
Stoney for magnitude of the charge, soon became associated
with the actual particles themselves.
Assuming that n particles each of mass m and velocity
v hit the collector in time T, and that each particle carries a
charge q, then Q = nq (1) provided each particle "sticks"
to the collector, and hence deposits its charge to be measured,
and does not cause any secondary emission of charged particles.
Thomson tried to ensure that these conditions would be met by grounding
his shielding electrode. (Even so, if the collector acquires a negative
potential from the incoming beam, then it is possible that some
of the beam may bounce away from it and be collected by the shield.)
Further, if each particle is collected it will
relinquish its kinetic energy to the collector, and this energy
will be evident in the form of heat:
H = n(1/2mv2) (2)
Dividing Equation (1) by Equation (2) one finds
q/m = 2Qv2H (3)
It is hence necessary to measure velocity v.
Now, in a magnetic field oriented so as to be
perpendicular to the original path of the rays, the resulting path
is observed to be part of a circle. The magnetic field, then, must
be exerting a centripetal force upon the ray particles. Assuming
that each of the particles has a mass m, a velocity v, and charge
q, and that they move in a magnetic field of intensity B in a path
of radius of curvature R, one can write the following equations:
Magnetic force on particle = Centripetal force
for circular motion
Bqv = mv2/ R (4)
Which can be rearranged to give
q/m = v/BR (5)
Hence, combined with magnetic field deflections,
the relations (3) and (5) would give values for both q/m and v.
Using this method, Thomson found values of v of the order of 2.4x107
to 3.2x107 meters per second (about one tenth the velocity of light),
and from 1.0 to 1.4x1011 coulombs / kilogram for the ratio of charge
to mass for cathode ray particles.
In the same paper that described the method described
above, Thomson reported a different method for getting the needed
second relationship between q/m and v. In this second method he
used a tube popularly known as the Thomson tube. The schematic diagram
for determination of q/m is shown elsewhere. The cathode ray beam
could be sent through an electric field produced by the plates A
and B in which region there could also be a magnetic field perpendicular
to the paper established by external coils. Any deflection of the
beam could be measured by the scale S at the end of the tube.
Electron As A Wave
The diffraction and interference properties
of radiation necessitate a wave structure, but photoelectric
phenomena and the Compton effect imply that radiation consists
of particles rather than waves. In Compton effect, an instant
X-ray is scattered by a free electron just like in a collision
between two rigid spheres. In other words, radiation may be
regarded as exhibiting a dual wave - particle behaviour; some
of the properties of the radiation may be wave properties
where as others are particle properties. By means of Planck's
Quantum Theory equation and the mass energy relationship of
Einstein, Prince Louis-Victor Pierre Raymond de Broglie (1892-1987)
deduced that a particle mass m moving with a velocity v should
be associated with waves of length l (lamda), given by
l = h / mv
where h is the Planck's constant. It was
calculated that with a moderately high velocity such as could
be obtained by passage of an electron through a potential
of about 100 to 1000 volts, the de Broglie waves should have
a wavelength of the order of 10-8 cms. If this were the case,
then crystals should be capable of producing diffraction effects
The first definite proof that electrons
can be diffracted and consequently exhibit wave, as well as
the familiar particle, properties was obtained in the Bell
Telephone Laboratories in New York by C.J. Davisson and L.H.
Germer in 1927. By studying the reflection and scattering
by a nickel crystal, of a beam of electrons, given a specific
velocity by passage through a known potential difference,
it was found that the electrons behaved like waves rather
than particles. Using electrons which had been accelerated
by a potential of 54 volts, the experimental results were
found to be equivalent to those expected from radiation of
wavelength 1.65 Å, in remarkably good agreement with
the value of 1.67 Å calculated by means of the de Broglie
Further, evidence for the existence of electron
waves was obtained independently in 1927, by George Paget
Thomson (1892-1975), son of J.J. Thomson. He passed a stream
of fast moving electrons through a very thin sheet of metal
and then allowed the resulting beam to fall on a photographic
plate. Upon development, the plate showed a diffraction pattern
consisting of a series of concentric circles, just as might
have been produced by X-rays, indicating that the electrons
were manifesting wave properties.
With no electrical field applied, the magnetic
field can deflect the beam upward or downward as shown, for example,
by the dotted path in the figure. The beam travels in a straight
line except in the region of the magnetic field, where its path
is an arc of a circle of radius R, as in Equation 5. Neglecting
the fringe effects, it is fairly easy to calculate R from the measured
deflection of the beam and the geometrical constants of the tube.
Thus this tube may be used, as other tubes had been, for measurements
with the magnetic field alone, and under these circumstances Equation
5 would apply.
But suppose that while the magnetic field is present,
a potential difference V is applied to the two deflecting plates.
If the plates are a distance D apart, then the resulting electric
field strength between the plates will be
E = V/D (6)
(If V is measured in volts and D is measured in
meters, then E will be found in newtons per coulomb). A charge q
in this field experiences a force Eq, up or down depending on the
sign of q and the direction of the field E. If we arrange the applied
potential difference to have a value such that the force Eq is numerically
equal, but opposite in direction, to the force on the particles
due to the magnetic field (which is Fmag = Bqv, in which v is the
velocity of the particles), we would then have
Felec = Fmag
or Eq = Bqv and hence (7)
v = E/B (8)
(It is easy to show that if E is measured in newtons
/ coulomb, and B in webers per square meter, then v is in meters
per second). In practice, the potential V is varied until the beam
is observed to be at a position of no net deflection. Zero deflection
implies that the electric and magnetic forces are equal. V is then
measured with a voltmeter, and B is determined by use of a search
coil and ballistic galvanometer. One can then put the numerical
value of v thus found back into Equation 5 to determine the value
of q/m. Thomson found, in a series of experiments, values for q/m
which, when averaged, came to 0.77x1011 coulombs / kilogram. This
value disagreed with the one he published from his heating effect
experiments, a disagreement he attributed primarily to possible
systematic errors in the latter experiments. In his writings for
the next few years he usually gave the value of q/m as "approximately
1011 coulombs / kilogram".
Thomson boldly announced the hypothesis that "we
have in the cathode rays matter in a new state, a state in which
the subdivision of 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". Thomson remarked that this
surprising result might be due to the smallness of m or to the bigness
of e. He argued that m was small, citing Philipp Lenard, who had
shown that the range of cathode rays in air (half a centimeter)
was far larger than the mean free path of molecules (10-5 cm). Lenard
was awarded the Nobel Prize in Physics in 1905 for studying the
cathode rays. If the cathode ray travels so much farther than a
molecule before colliding with an air molecule, it must be very
much smaller than a molecule. Thomson concluded that these negatively
charged particles were also constituents of atoms.
From 1897 onward, thanks largely to the experiments
of Perrin and Thomson, the corpuscular model for cathode rays received
general consent. Thomson's view that the cathode ray particles were
the fundamental building block, or even a fundamental building block
of atoms was not, however, received with much enthusiasm. Several
other lines of research, notably in the fields of analysis of spectra
and of radioactive phenomena, had to converge before the real role
of Thomson's corpuscles within the atom could be understood and
Thomson's achievements were honoured in numerous
ways, and mark him as among the most accomplished physicists of
his era. In 1906 he was awarded the Nobel prize in physics for his
researches into the discharge of electricity in gases. In 1918 he
was chosen Master of his old college, Trinity, and the next year
he resigned the Cavendish Professorship. He guided Trinity with
his usual common sense and benevolence until shortly before his
death in August 30, 1940.
by Millikan's method
Millikan's apparatus consisted of two horizontal
metal plates about 22 cm diameter and 1.6 cm apart as indicated
by A and B. The plates were supported in a closed vessel containing
air at low pressure, and were connected to the poles of a
high voltage (10,000 volts) battery, V. In the upper plate,
there were a number of small holes as represented by C. By
means of a atomizer, a fine spray of a non-volatile oil was
introduced into the vessel. As a result of friction in the
atomizer, the droplets of oil so obtained were electrically
charged. From time to time, one of these droplets would pass
through the hole C, and then it could be observed by means
of a telescope (not shown in the figure). By using the illumination
of a powerful beam of light, entering the window W (at left),
the droplet appeared as a bright star on a dark background.
With the battery V disconnected, the droplet
fell slowly under the influence of gravity, and the rate of
fall was measured. This rate (or velocity) represented by
v1 is dependent on the mass m of the droplet and is given
by the equation v1 = kmg where g is the gravitational acceleration
(981 cm/sec2) and k is a proporationality constant which is
related to the viscosity of the air and the size of the oil
droplet. The high voltage battery was then switched on, thus
producing the electric field, the direction being such as
to make the charged droplet move upward, against the force
of gravity. If E is the strength of the electric field, i.e.,
the voltage of the battery divided by the distance between
the plates, then the upward force acting on the droplet is
Een , where en is the charge carried by the droplet. Since
this is opposed by the gravitational force mg, the net upward
force is Een - mg. The upward velocity v2 of the oil droplet
which is measured, is then represented by
v2 = k (Een - mg)
The proportionality constant k has the same significance as
in the previous equation.
From the above two equations, it is easy to show that
en = mg (v1 + v2) / Ev1
Since the quantities v1, v2, g are available,
it is possible to calculate the charge en carried by the oil
drop if the mass m were known. Using Stokes Law, applicable
to small spherical drops falling under the influence of gravity,
it could be shown that
v1 = 2gr2d / 9h
Here h is the density of the oil of which
the drops are made. Since v1 has been determined, as described
above and g, h and d may be regarded as known the radius r
of the drop could be determined from the above equation. It
is now easy to determine the mass of the oil drop which is
given by m = 4pr3d / 3, inserting this result into the equation
for en, together with the measured velocity, v1 and v2, the
magnitude of the charge en carried by the oil droplet can
now be determined.
As a result of a large number of measurements,
Millikan found that the charge en was always an integral i.e.
a whole number, multiple of a definite elementary charge,
which was presumably the electronic charge. After applying
numerous corrections to the foregoing equations, Millikan
concluded in 1917 that the most reliable value of the unit
charge was 4.774 X 10-10 esu which is very close to the modern
value 4.803207 X 10-10 esu.
Millikan and his oil drops
Robert Andrews Millikan (1868-1953) was the son
of Silas Franklin Millikan a congregational preacher, and Mary James
Andrews, a graduate of Oberlin who had been Dean of Women at a small
college in Michigan. Raised in Maquoketa, Iowa, where his family
moved in 1875, young Millikan enjoyed a story book, Midwestern American
boyhood, fishing, farming, fooling and learning next to nothing
about science. In 1886, he enrolled in the preparatory department
of Oberlin College and in 1887, in the classical course of the college
itself. At the end of his sophomore year, he was asked to teach
an introductory physics class. Millikan plunged into the subject,
liked it and soon decided to make it his career.
Millikan graduated from Oberlin in 1891 and continued
to teach physics to the preparatory students. He was awarded an
M.A. for his achievement of successfully pursuing a course of instruction
in Dynamic Electric Machinery in 1893. Millikan entered Columbia
University on a fellowship as the sole graduate student in physics.
He was impressed by the experimental deftness of Michelson, under
whom he studied at the University of Chicago in the summer of 1894.
After receiving his Ph.D. in 1895, Millikan went to Europe for post-graduate
study. He heard PoincarŠ lecture at Paris, took a course from
Planck at Berlin, and did research with Nernst at Gottingen. In
1896, the excitement of the discovery of X-ray still fresh in his
mind Millikan joined the faculty of the University of Chicago as
an assistant in physics. This is where he met Greta Irwin Blanchard,
the daughter of a successful manufacturer from Illinois whom he
married in 1902. He spend a large fraction of his energies into
the development of the physics curriculum, especially in introductory
courses and wrote/co-authored a variety of text books which quickly
became standards and sold in large numbers. Mainly because of his
outstanding pedagogical achievements, Millikan was promoted to an
By 1909 Millikan was deeply involved in an attempt
to measure the electronic charge. No one had yet obtained a reliable
value for this fundamental constant, and some any-atomistic continental
physicist were insisted that it was not the constant of a unique
particle but a statistical average of a diverse electrical energy.
His famous experiment of determination of the electronic charge
is described elsewhere in the article. Of and on all the while Millikan
had continued his exploration of the photoelectric effect and by
1950 had confirmed the validity of the Einstein's equation of photoelectric
effect in every detail.
Millikan held many important posts and membership
of several eminent Academies and Societies. In 1921, Millikan accepted
appointment as the Chairman of the Executive Council and Director
of the Norman Bridge Laboratory at the California Institute of Technology.
Employing the photonic interpretation of cosmic rays, Millikan developed
a theory of their origin in 1928. To find a measure of cosmic ray
energies, he put Carl Anderson, a young research fellow at Caltech
to work with a cloud chamber set in powerful magnetic field which
ultimately lead to which detection of the negative electron - also
called Positron, in 1932.
How the "Positive Electron"
or the "Positron" was Discovered
The English mathematical physicist P.A.M.
Dirac (1902-1984) in 1928 presented theoretical arguments
indicating that a particle similar in mass to the electron
but carrying a positive charge may exist. His discussion based
on relativistic wave mechanics was of a highly abstruse character.
However, the proof of the existence of the long-sought positive
electron was obtained by C.D. Anderson (1905-1991) at the
California Institute of Technology in 1932. In order to study
the so-called cosmic rays, which appears to come from outer
space, Anderson in conjunction with R.A. Millikan, had constructed
an apparatus known as a cloud chamber which was placed in
a very strong magnetic field. In this cloud chamber, the path
of an electrically charged particle could be rendered visible
and also photographed. A cloud chamber is based on the fact
that whenever an electrically charged fast-moving particle
passes through the air (or any other gas), it produces ionization
along its tract. If the air through which these particles
pass is saturated with water vapour, the ions serve as the
centres of condensation for tiny water droplets, and we see
long thin tracks of fog stretching along the particle's trajectories.
The intensity of the track provided information concerning
the mass of the particle, and the direction in which it was
bent in the magnetic field indicated whether the charge was
positive or negative. Numerous tracks were observed due to
charged particles resulting from the impact on matter of the
very highly energetic cosmic rays. A lead plate of 6 mm thickness
was placed across the chamber with the object of depriving
the particles of some of their energies. Anderson stated in
one of his lectures: "The degree of curvature in the
magnetic field shows a difference depending on the amount
of energy lost in the plate. Measurements made on the track
of a particle before and after it has passed through the plate,
together with observations of the density of the track itself,
give definite information about the mass of the particle and
the magnitude of the electric charge it carries" (Figure).
The photograph shows one of the numerous
photographs obtained in this manner - a photograph of historical
significance, for its interpretation by Anderson lead to the
discovery of the positive electron. Since the curvature of
the track is less below the plate than above, the energy of
the particle is greater below the plate. Hence the particle
must have been moving upward. Knowing the direction of the
magnetic field and the direction of the motion of the particle,
the curvature of the track to the left immediately showed
that the particle must be positively charged. The density
of the track was less than would be expected for a proton,
but its length was greater. "Photographs of these positively
charged particles could be understood only if the particles
were assumed to have a mass approximately equal to that of
the ordinary electron of negative electric charge, and thus
the first evidence for the existence of the positive electron
.... was obtained", Anderson said.
Other cloud chamber photograph examined
in the light of new discoveries provided further proof that
positive electron were produced by the action of cosmic rays.
Some photographs showed charged particles to fall into two
groups, one being deflected in one direction and the other
in the opposite direction by the magnetic field, representing
negative electrons and the positive electrons respectively.
Anderson suggested the name "Positron" for the positive
electron, and this immediately became into general use.
Millikan was an able populariser and lecturer
and after he won the Nobel Prize in 1923, he became perhaps the
most famous American scientist his days. He was an outspoken, religious
modernist. Even after his retirement from his professorship in 1946,
he remained active as a public lecturer and spoken frequently on
the subjects of science and religion. By the time of his death,
he had been awarded numerous medals even from honorary degrees and
professorship of 21 foreign scientific Societies, including the
Royal Society of London and Institut de France.
Thomson did not use the term "electron"
to refer to his negatively charged particles; he preferred the term
"corpuscle". "Electron" had been introduced
by the Irish physicist G.Johnstone Stoney in 1891, as the name of
the "natural unit of electricity", the amount of electricity
that must pass through a solution to liberate one atom of hydrogen.
Stoney did not associate the electron with a material particle,
and physicists at the time questioned whether or not electricity
might be a continuous homogeneous fluid. (Figure)
The early determinations of the charge of the
electron had not established that there was a fundamental unit of
electricity. That was because the experiments measured the total
charge of a cloud of droplets, without showing that the value obtained
was anything other than a statistical average. The same was true
for Thomson's measurement of e / m for a beam of cathode rays.
It was the experimental work of Robert Millikan
at the University of Chicago, beginning in 1909, that provided the
next step in establishing the electron as a fundamental particle.
Millikan not only demonstrated that there was a fundamental unit
of electrical charge; he also measured it accurately.
Millikan's experimental apparatus and the method
he used for the determination of the electronic charge is described
in a box. He allowed single oil drops to fall a known distance in
air, and measured the duration of the fall. He then turned on an
electric field and measured the time it look for each drop to travel
the same distance upward. (The oil drops were travelling at constant
terminal velocity). These two time measurements let him determine
both the mass of the drop and its total charge.
The charge on the oil drop sometimes changed spontaneously,
by ionization or absorption of charge from the air. Millikan also
induced such changes with either a radioactive source or x-radiation.
One could calculate the change in the charge on a drop and the changes
in that charge were small integral multiples of e, a fundamental
unit of charge.
Millikan wrote, "The total number of changes
which we have observed would be between one and two thousand, and
in not one single instance has there been any change which did not
represent the advent upon the drop of one definite, invariable quantity
of electricity or a very small multiple of that quantity".
Millikan's final value for e was (4.774 ± 0.009) x 10-10
esu. (The modern value is 4.803 207 x 10-10 esu).
Millikan associated his measured e both with the
charge on Thomson's corpuscles and the charge on the hydrogen ion
in electrolysis. He combined his value for e with contemporary measurements
of e / m by electrolytic and cathode ray techniques to determine
that the mass of Thomson's corpuscle was 1/1845 that of the hydrogen
atom surprisingly close to 1/1837.15, the modern value. Now one
had both a definite mass and a definite charge for this would be
fundamental particle, and it behaved exactly as one would expect
a negatively charged particle to behave. There was now good evidence
for believing that it was a constituent of atoms in other words,
the electron. He was awarded the Nobel Prize for his work in 1923.
Electron gets established
1. The Zeeman Effect
After Hertz had shown experimentally that electromagnetic
waves are, in fact, produced by oscillating electric charges as
predicted by Maxwell's equations, it became commonly agreed that
light waves were electromagnetic waves and that they were due to
some sort of oscillation of charged particles within, or associated
with, molecules or atoms. In 1896 a Dutch physicist, Pieter Zeeman
tried to see whether an external magnetic field would affect the
wavelength of the light given out by these hypothetical oscillators.
His apparatus was, in principle, quite simple: a light source (for
example, sodium vapour in a gas flame) was placed between the pole
faces of an electromagnet, and the light from the source was sent
through a spectroscope. In his attempts Zeeman found that the spectral
lines were not changed or shifted when he switched on the magnet.
He gave up the experiment, but then happened to read Faraday's accounts
of his final experiments some forty years before. He found that
Faraday had tried essentially the same experiment. Zeeman's admiration
for his predecessor was so great that he decided that if Faraday
had thought the experiment worth doing, then he, Zeeman, ought to
be willing to put in a little extra effort to repeat it. With a
somewhat stronger magnetic field he found that the spectral lines
(he was using the well known "D lines" of sodium vapour)
were slightly broadened. The broadening was of the order of one
fortieth of the separation between the two lines, or about 0.15
Angstrom Unit (Angstrom unit = 10-10 metre).
A few years later, it turned out that if one assumes
that the emission of light takes place from small charged particles
revolving in orbits in the atoms, then one can predict a slight
contraction or expansion of the orbits when an external magnetic
field is applied. The expansion or contraction of the orbits would
result in a slight shift in the wavelength of the emitted electromagnetic
radiation. The actual amount of the shift can be predicted if one
knows the ratio of q to m for the orbiting particles and the strength
of the magnetic field. Lorentz and Zeeman were thus able to postulate
the presence within the atom of small charged particles, and to
estimate from the line broadening that the ratio of q to m for these
particles would have to be about 107 emu per gram. This was, as
they were quick to point out, the same ratio that Thomson had found
for his cathode ray particles. Lorentz and Zeeman were jointly awarded
the Nobel Prize in 1902 for their work.
2. The Photoelectric Effect:
Hertz in his famous 1887 experiments with electromagnetic
radiation had made what seemed like an incidental or an accidental
discovery, which was that ultraviolet light falling on certain metals
caused them to emit negatively charged particles, or what is known
as the photoelectric effect.
The discovery of the electron at once suggested
the hypothesis that the photoelectric effect is due to the liberation
from the illuminated metal plate of electron which under the influence
of the electric field pass from cathode to anode, thereby causing
photoelectric current. This hypothesis was confirmed by Lenard who
showed that the photoelectric discharge is deflected in a magnetic
field exactly as are cathode rays. By measuring the deflection of
the "photoelectric rays" in a known magnetic field, he
found a value of e/m about 1.2x107 in qualitative agreement with
Thomson's value of e/m for electrons.
In 1899 Thomson applied the technique that he
had used with photoelectrically emitted particles to a determination
of q/m for the negatively charged particles that, as Edison (the
same Thomas Alva Edison) had discovered, are emitted by white hot
metals. Thomson found q/m for these particles to be 0.87x1011 coulombs
per kilogram, again in satisfactory agreement with his value for
cathode ray particles. In the next few years Owen in England and
Wehnelt in Germany found similar values for particles emitted by
certain metallic oxides heated to a red heat.
Lenard found out in 1902 that there was no relationship
between the intensity of the light and the energy of the electrons
emitted. And a brighter light might cause more electrons to be emitted,
but they would not be any more energetic than those released by
a dim light. Classical physics could offer no explanation.
By the turn of the century it was known that certain
radioactive materials emitted negatively charged particles that
had come to be called "beta rays". In 1900 Becquerel sent
a beam of such particles through electric and magnetic fields to
determine their velocity and their ratio of charge to mass. He found
the then rather astonishing velocity of approximately 2/3 that of
light, and a ratio of charge to mass of about 1011 coulombs per
kilogram. Kaufmann, in 1901 and 1902, determined q/m for beta rays
more precisely, finding it to be 1.77 x 1011coulombs per kilogram.
That's where Einstein stepped in, breaking out
Planck's quantum theory, which had been gathering dust for a couple
of years without too much attention. Planck had pointed out that
light emits distinct "packets"; Einstein added that light
also travels in packets. Einstein pointed out that a particular
wavelength of light is made up of quanta of fixed energy content,
according to quantum theory. When a quantum of energy bombards an
atom of a metal, the atom releases an electron of fixed energy content
and no other. A brighter light would contain more quanta, still
always of fixed energy content, causing the emission of more electrons,
also still all of the same energy content. The shorter the light's
wavelength (and the higher the frequency), the more energy contained
in the quanta and the more energetic the electrons released. Very
long wavelengths (of lower frequency) would be made up of quanta
having much smaller energy content, in some cases too small to cause
any electrons to be released. And this threshold would vary depending
on the metal.
This was the first use of Planck's theory since
its invention to explain the blackbody problem - and once again
it succeeded in explaining a physical phenomenon where classical
physics could not. For this work, Einstein received the 1921 Nobel
Prize in Physics. It was the first major step in establishing what
would become known as quantum mechanics, the recognition of the
discrete and discontinuous nature of all matter, especially noticeable
on the scale of the very small.
All paths lead to Rome:
Thus within four or five years of Thomson's 1897
investigations, he and others were able to show that electrons,
as they were then commonly called, with essentially the same properties
are emitted from all sorts of materials by several different mechanisms:
(a) By strong electric fields, or by bombardment
of the cathode by positive ions, as in the classical cathode ray
(b) As a result of absorption of ultraviolet light by atoms.
(c) By thermal agitation of atoms in white hot metals or oxides.
(d) By some spontaneous process within radioactive atoms.
In addition, the Zeeman effect could best be interpreted
as showing that precisely similar particles exist within atoms.
(This is of some significance, because it is not logically necessary
for an atom to "contain" some particle that it is later
observed to emit. Modern physics abounds with examples of emitted
particles that are produced, as it were, on the occasion of their
Thus by 1900 the electron was well established
as a constituent of atoms. Already physicists were working toward
a better knowledge of the electron's inherent characteristics, its
charge, mass, and size, and toward an understanding of its role
in an astonishingly wide array of chemical and physical phenomena.
But follow the story further we must.
Electron through the 20th Century:
In 1913, not long after Millikan's oil drop results,
Niels Bohr constructed a theory whose confirmations provided support
for the view that the electron was both a fundamental particle and
a constituent of atoms. Bohr developed his theory based on Rutherford's
nuclear model of the atom put forward in 1911 (in contrast to his
mentor Thomson's idea that the positively charged particles in the
atom were spread like a fluid throughout the atom _ a sort of plum
pudding!). Rutherford's model of the atom had a small, massive positively
charged nucleus orbited by electron of mass in and charge -e. A
decade later, Bohr's theory was superseded by the Quantum Mechanics
of Erwin Schrodinger and Werner Heisenberg which also assumed an
electron with charge e and mass m and it gives exactly the some
predictions as the Bohr theory for the Balmer series in the Hydrogen
In the 1920, the experiments of Otto Stern and
Walther Gerlach established the existence of the spatial quantizations
which provided the evidence for an intrinsic spin of the electron,
that is, it behaves as though like a tiny spinning top and a magnet
of certain strength.
The electron itself has turned out to be not quite
the creature that J.J. Thomson thought it was. According to the
quantum theory developed by Albert Einstein and others, it is a
mistake to think that electrons must be either particles or waves
but not both. Under some conditions electrons act like particles;
under other conditions they act like waves. (The wave character
of electrons was in fact experimentally indicated by J.J. Thomson's
own son, G.P. Thomson, who as a result shared the Nobel Prize in
1937). Physicists have also found that electrons are only the most
common members of a whole "family" of related fundamental
particles - - all of them infinitesimal points carrying charge,
mass, and something called "spin". Why the particles have
these properties remains a mystery, a grand challenge for the next
century of research.
The knowledge we have gained has made key modern
technologies possible. When you are sitting in front of a computer
monitor or watching television, you are probably looking at a direct
descendent of the cathode ray tube that Thomson used in his 1897
experiments. Other solid state devices also descend almost as directly
from the discoveries of Thomson and his colleagues. Indeed most
of our civilization's computation, communications, entertainment
and much else rely on technical calculations that would have been
impossible without knowledge of the electron and its properties.