The quantum world is a very strange place and you may have read some of the pages in this section and
found them hard to follow! This is the nature of quantum physics, however, as these pages develop we shall
build up a more complete picture and endeavour to provide more accessible explanations. Persevere and
you shall be rewarded! In this page we discuss the electron, one of the fundamental sub-atomic particles and
a major constituent of the atom. The approach we use will be one of following a series of experiments and we
shall see what is done, note what is observed and conclude what it all means!
Experiment 1: Thermionic Emission
For this demonstration we shall use the device below which superficially resembles a light-bulb. This is a
thermionic emission tube.
The device has two pins at the back with which we can connect it to an electrical power supply. It also has a
filament and a metal plate at the front which is connected via a metal pin and can be connected to an
electrical circuit. As with a normal light-bulb, the air has been removed from the bulb (to prevent the filament
from burning when hot and to prevent air from impeding or scattering the electrons). We connect the tube to
a 400 volt (400 V) power supply and use a galvanometer to measure the electrical current in the circuit, as
Notice that the switch is open, breaking the circuit. Electricity will only flow around a completed circuit. The
black lines represent our electrical cables and notice that the galvanometer is the metre reading 0 to 2 mA. A
milliamp (mA) is one thousandth of an Amp. An Amp (Ampere) is a measure of electrical current. Electrical
current is the flow of electric charge. Notice that the metre is connected to the plate at the front of the tube,
this is the target plate. The picture below shows the tube switched on.
The electrical current flowing through the filament makes the filament hot and it glows brightly, as the
resistance in the wire impedes the flow of electric charge and turns some of the electrical energy into heat
and light. Nothing surprising so far, however look at our galvanometer below:
The meter now reads almost 2 mA. The meter is not connected across the power supply, clearly we expect
current to flow there, but to the target plate. Remarkably an electric current is flowing from the filament to
the target plate. Some sort of particle is carrying electric charge across the space inside the bulb, carrying
electric current between the filament and target plate, even though no wire connects them. Notice that the
target plate carries a positive electric charge, we know this since we connected the galvanometer to the
positive terminal of the power supply. Now, here is the crucial bit - if we disconnect the lead from the
galvanometer and the power supply and reconnect it to the negative terminal of the power unit, thus
making the target plate negatively charged, then no current flows and the galvanometer falls to zero again.
Likewise, when the filament is cool (the circuit is switched off) no current flows.
From this observation we conclude that the particles carrying electric charge from the filament to the plate
are negatively charged, since like charges repel and opposite charges attract. Thus when the target plate
is positive the negative charge carriers are pulled towards it and current flows. This makes the filament
negatively charged as shown. The negatively charged terminal (filament) is called the cathode and the
positively charged terminal (target plate) the anode.
What are these negative charge carriers? Could they be particles of light, called photons? No, because
photons are neither negatively nor positively charged, they are neutral (have zero electric charge). Some
other type of negatively charged particle is being emitted from the hot filament - a process called
thermionic emission. We call our charge carriers electrons, since they carry electricity between the
cathode and anode.
Since these negatively charged electrons are derived from the metal of the filament by heating it, we
conclude that they are a component of the metal atoms. Since metal is normally neutral (it has no net
electric charge) we also conclude that the atom must have a positively charged component which we
separated from the electrons.
Thermions: thermion is a general name given to any electrically charged particle emitted by hot metal.
Thermions include electrons and possibly also the positively charged parts of the atom, which we call an
ions. However, in our experiment there are clearly not many ions liberated, otherwise we would get current
flow when the target plate is negatively charged, as the ions will be drawn toward it. A hotter metal may
liberate positive ions as well as electrons, however (depending also on the metal used). In fact, even at
room temperature, some electrons escape from the surface of metal, since some electrons move about
very fast within the metal. However, these electrons do not have enough energy to escape completely and
fall back to the surface of the metal, since when an electron is released, the metal becomes overall
positively charged and so attracts the electron back, and so the electron is launched from the metal and
then falls back down like a projectile falling back to ground. However, when we apply a voltage (potential
difference) across the metal, as we did in our experiment, the electrons can be pulled off toward the anode
and as the circuit is completed, these electrons are continually replaced.
Electrical current: we now know that electric current flowing down a metal wire is the movement of electrons
from the atoms of the metal. Metals conduct electricity because they have loosely bound electrons that are
free to move throughout the metal. By applying voltage from a power supply, such as a battery, these
electrons can be repelled (pushed) by the negative terminal and attracted (pulled) toward the positive
terminal of the battery and will flow along any metal that bridges the two terminals. Before it was discovered
that electrons carried electrical current in wires, electrical current was defined by convention to flow from
positive to negative terminals, as it would if positive ions carried it, but it actually flows from negative to
positive, however the convention has stuck and, in the end, positive current flowing in one direction is
equivalent to negative current flowing in the opposite direction anyway! The positive ions are not mobile,
since they constitute the bulk of the atom and are much heavier than free electrons.
Conclusion: negatively charged particles, called electrons, are emitted from the hot filament.
Experiment 2: The Maltese cross tube
The Maltese cross tube is shown below:
We have a similar arrangement as before, except that now we have a fluorescent screen at the front of the
tube, instead of the anode plate. This screen will glow bright green when electrons hit it. We have
introduced a metal cylinder between the filament and the screen, this is now the anode. This is a special
anode - it is a hollow cylinder open at the end that faces the filament, and with a narrow hole in the other
end. This acts as a collimator, converting the electrons radiated by the hot filament into a narrow beam
(column) of electrons. The anode is attached to a positive terminal, making it positively charged, so that it
attracts the electrons to it, and the electrons then whiz on through the aperture and leave the anode,
spreading out as it does so. This electron beam (or cathode ray) leaves the anode and is drawn toward
the metal cross which is also positively charged, but what happens next? (Note originally a Maltese cross
was used, but I have used an equal-armed simple cross since that is easier to make on Pov-ray).
The diagram below shows the circuit we are going to use (switched off at the moment):
When we switch on the circuit and heat the filament, it emits light (photons) and electrons as before (see
experiment one, thermionic emission). However, we see a shadow of our cross on the flourescent
screen, as electrons that pass the cross form a bright image, but those that hit the cross are stopped.
Below: another view, showing the aperture in the anode from which the cathode rays emerge. (These
images were generated in Pov-Ray and Pov-Ray generated the shadow for us).
Conclusion: electrons travel in straight lines (they are rays - cathode rays) and are stopped by a thin sheet
of metal (the cross). Light also travels in rays, which enables them to form optical images, in much the
same way electrons have been used to make an image (shadow) here.
The effects of magnetism on cathode rays
Now, if we bring a magnet up to the side of the tube, the shadow moves! This demonstrates that the
shadow is caused by moving particles carrying electric charge, since moving electric charges are deflected
in a magnetic field. (Actually, there are two shadows - one created by electrons which deflects, and one
created by photons which does not get deflected by the magnet). Furthermore, the direction that a beam
of charged particles gets deflected, depends upon the sign of the electric charge on the particles - if they
are positively charged then the beam deflects one way, but if they are negatively charged then it deflects
in the opposite direction. In fact, if we place the North pole of a magnet to the right of the tube, with the
electron beam coming toward us (i.e. with the screen facing us) then the shadow will move up. This shows
that the electrons are negatively charged.
Caution: when electrons strike matter, such as the fluorescent screen, they emit harmful X-rays, so don't
look too close!
Experiment 3: The Thomson cathode ray tube as used to measure electron velocity
The Thomson tube is shown below. The bulb at the back end (right end) contains a metal plate, which is
our cathode now (instead of a filament). The cathode rays are channelled into beams by two collimators
(anodes) which are the two metals discs inside the narrow part of the tube (these are similar to the anode
in the Maltese cross tube). So, when switched on, we get a beam of cathode rays produced, which then
passes between two horizontal metal plates in the wide part of the tube. These plates connect to wires and
can be connected to terminals, allowing an electric field to be applied across them. The beam passes
between these plates and continues on to hit the fluorescent screen.
This time, when we switch the tube on, we get a narrow spot of light appear on the flourescent screen.
Now, if we apply an electric current across the two horizontal metal plates, making the top one negatively
charged and the bottom one positively charged, then the spot moves down. Reversing the polarity,
making the bottom plate negative and the top plate positive, the spot moves up, as shown below, the
blue arrow indicates the path of the electron beam:
Now, here is the really clever bit. If we apply an electromagnetic (such as a pair of Helmholtz coils) to the
side of the tube, then as with our Maltese cross experiment, we can deflect the beam, moving the spot
back to the centre of the screen - we can use the magnetic field to exactly cancel the electric field. Now,
the key is this: the deflection of the beam caused by the electric field depends only upon the electric
charge of the electron and the strength of the applied electric field, whereas the deflection caused by
the magnetic field depends upon the charge of the electron, the strength of the magnetic field AND the
speed of the electrons. Since we know the strengths of both the electric and magnetic fields (we applied
them, so we can control them) and the electron charge is constant, we can calculate the speed of the
electrons in the beam! By using either the electric field or the magnetic field alone, once we know the
electron velocity, we can use the laws of physics and some mathematics to find the ratio of electric
charge to mass of a single electron! This demonstrates that electrons are discrete units with definite
mass, that is they are particles of matter. (Albeit very tiny with no measurable diameter).
Thomson did this and found that the electrons moved at about 10% the speed of light, or about 30 000
kilometres per second! This experiment was first done by J. J. Thomson in 1897 and proved that
cathode rays are rays of particles of matter called electrons - the electron proper was discovered.
Conclusion: electrons can be deflected by an electric field, and when moving by a magnetic field.
Electrons can move very fast indeed! Electrons are tiny particles of matter that make up cathode rays.
Application: the cathode ray tube
The above principles have been incorporated into an extremely common device - the cathode ray tube,
found in TV sets and computer monitors (except the new flat screen devices). It is also used in
oscilloscopes in laboratories (being replaced by computers nowadays). The cathode ray tube (CRT)
consists of three principal parts: the electron gun, the deflecting system and the flourescent screen.
Experiment 4: Electron diffraction
So far we have talked about electrons as 'particles' or tiny point-like objects (or minute balls if you prefer)
that have mass and behave as definite solid entities, all be it so small as to be individually invisible.
However, our view is about to change!
The picture below shows an electron diffraction tube, it is very similar to the Maltese cross tube and has a
collimating anode, but has no cross, and so the electron beams spreads out unimpeded and strikes the
fluorescent screen, producing a large bright spot.
So far, nothing surprising. However, we shall now introduce a target, a thin sheet of material, in the way
of the electron beam. We place this thin sheet inside the anode (but it could be placed inside the bulb).
This sheet must be thin enough to allow some electrons through, it could be foil or a slice of graphite.
Now what we get is shown below:
Surprisingly, we get a series of bright and dark rings on the screen! This is called a diffraction pattern.
Light waves and water waves also diffract, indeed any type of waves diffracts. However, solid particles
were not hitherto expected to diffract - fire a series of bullets through a target and if some get through
then you still get the same pattern on a screen at the other side (the bullets may deflect or scatter and
emerge at odd angles, but they certainly would not form a fancy pattern of circles!). So now the
electrons are behaving like waves rather than particles. This schizophrenic behaviour is peculiar to tiny
things and is called wave-particle duality. Photons can be diffracted like waves, but can also behave like
particles, as can electrons and even atoms and molecules, but a football is way too large to exhibit
wave-like behaviour! Let us take a closer look at diffraction in waves (though this idea is a huge topic
and shall be considered in depth elsewhere).
The image below shows a (simulated) ring diffraction pattern, generated in Pov-Ray by using a glass
ball to diffract light (Pov-Ray doesn't have 'real' diffraction apparently, but it often creates similar effects)
and just so happens to be practically identical in appearance to some of the diffraction patterns
obtained with electrons! How many rings can you see?
The electron gun consists of a heated cathode (the hot wire (H) heats the cathode disc (C)). The electron
beam passes through two collimators, the first one is negatively charged and the second one positively
charged. (There may be two or three of the second type in a row). The negatively charged collimator, is
called a grid (G) and repels electrons back to the cathode and can be adjusted - turning it up dims the
brightness of the beam and so this is our brightness control. The second acts as a usual anode (A) and
focuses the electrons into a narrow beam. The deflecting system contains a pair of electrically charged
horizontal metal plates (the Y plates), as before, which can move the beam vertically up and down, but also
a pair of vertical metal plates (the X plates) that can move the beam horizontally. Now, the beam can be
directed to any point on the flourescent screen. By scanning each row at a time (say from left to right and
from top to bottom) a picture can be generated. If a picture is built up in this manner say 50 times a
second, then the images will move too fast to be discerned as still pictures and a movie can be displayed!
If you place a magnet next to a TV screen, then you will see it distort the electron beam (and change the
colours on a colour TV), but be careful, these effects can sometimes last quite a while until the
magnetisation wears off!
If we want to be more accurate, we need a computer program to plot the mathematical diffraction pattern
expected. We have written such in-house software and some of the results can be seen on the waves
page. A fuller explanation of diffraction will appear in a future update. However, it happens when waves
pass an obstacle, or a gap in a wall, causing the waves to bend into semicircles. When electrons pass
through the foil or graphite target, they behave like a wave and these waves get bent as the electrons
pass between the atoms in the solid target. As these waves emerge on the other side, they overlap and
interfere with one another. Wave interference is the result of what we call linear superposition - two or
more waves can be added together to give either a stronger wave (constructive interference) or a
weaker wave or now ave at all (destructive interference). Where the electrons add constructively, the
screen fluoresces brightly and where waves add destructively, the screen is dark. This produces a
pattern of light and dark rings. These patterns can be complex and depend upon the arrangement of
atoms in the solid target. G. P. Thomson (the son of J. J. Thomson) won the Nobel Prize in 1937, along
with Davisson, for electron diffraction experiments similar to this one.
Diffraction patterns can also be obtained using photons (particles of light) and X-rays (rays of very high
energy photons) are used to determine the structure of crystals from the diffraction pattern - since the
pattern depends on the arrangement of atoms in a solid, it is possible to work backwards and obtain the
atomic or molecular arrangement of a solid from a diffraction pattern. Similar diffraction patterns have
been seen with neutrons, atoms and even molecules. It is an inescapable conclusion that matter is
made-up of waves that sometimes also behave as particles.
Conclusion: electrons can exhibit wave-like as well as particle behaviour and can be diffracted just like
waves on water or waves of light.
Coming soon ...
We will look at more experiments with electrons, including experiments using high energy particle
accelerators to produce very high energy electron beams!