Many scientific instruments rely on the ability to detect photons one
at a time, from the huge tanks of water built deep underground to detect
neutrinos from the Sun to the fluorescence detectors used in biological
research. Traditionally, this could only be done with photomulti-plier tubes
which are bulky, fragile and expensive.

But now a team of Finnish researchers has built a prototype semiconductor
chip that can count individual photons. The researchers have built a device
known as an avalanche photo-detector (APD), which in itself is not new,
but they have refined the structure of the device to make it accurate enough
to pick up photons one by one.

APDs detect photons because when a photon strikes the top surface of
the chip, its energy knocks an electron into an energy level in which it
can move around freely. Electrons in such a semiconductor can only exist
at certain energies which are divided into bands separated by forbidden
energy gaps. Electrons can only move around in the high-energy conduction
band.

Once a photon has freed up an electron, an electric field accelerates
the electron downwards into the ‘avalanche’ region. This is made up of extremely
thin alternate layers of semiconductor and a more insulating material. The
semiconductor layers are so thin that in every layer the lower energy bands
are split up into a few separate energy levels, each of which can hold only
a few electrons. These thin layers are known as quantum wells.

When the freed electron arrives at the first quantum well, it knocks
another out into the conduction band. These two then knock another two out
from the second quantum well. This continues from well to well creating
an avalanche of electrons which is large enough, by the time it reaches
the bottom of the chip to be detected. By careful construction of the avalanche
region, it is possible to build an APD which generates an output, or gain,
of 1500 electrons for every photon which is absorbed.

The problem, however, is that every electron knocked into the conduction
band leaves behind a hole. These holes can also move around freely and will
be accelerated by the electric field upwards towards the absorption region.
The holes create electrical noise in the chip which makes it difficult to
pick out the avalanche signals.

Markus Pessa and his colleagues at the Department of Physics of Tampere
University of Technology in Finland, have taken a major step towards overcoming
this problem. They have built a prototype APD with a multiple quantum well
made of layers of gallium arsenide and aluminium gallium arsenide. While
constructing it, they doped the quantum well layers with precise quantities
of impurities. These add excess electrons to the material so that fewer
holes are created during the avalanche of electrons and so noise is greatly
reduced.

At the moment, their device only works properly at extremely low temperatures,
but Pessa says that it shows that it will be possible to make commercial
photodetectors which are compact and have a low power consumption.

The new rules will outlaw the long-established practice of venting hydrocarbon
vapours straight into the atmosphere while oil is being loaded at terminals
a few hundred metres from the shore. These vapours help to form smog, and
contribute to other environmental problems.

Operators of terminals will in future be required to collect the vapours
and pipe them to the shore for incineration. This could be hazardous, because
air and oil vapour form an explosive mixture. There is a risk that the mixture
could ignite under the intense pressures and temperatures generated within
pipelines, to send flames surging forward at up to 4000 metres a second.

To prevent this happening, oil operators must fit vapour recovery pipelines,
which are 44 centimetres in diameter, with systems called ‘flame arresters’.
The arresters usually take the form of porous metal elements, produced by
cramming crimped metal ribbon or wire mesh into the core of a pipe. These
systems quench flames as they penetrate the cavities of the arrester element.

Unfortunately, no existing flame arresters are sturdy enough to cope
with the extreme stresses of the proposed vapour recovery pipelines. No
one has even calculated accurately the pressures and temperatures generated,
so no specifications exist for flame arresters that will be suitable in
this context.

However, some experimental systems have been developed, and it is these
systems that are under evaluation in test rigs at the Health and Safety
Executive’s Explosion and Flame Laboratory in Buxton, Derbyshire. The tests
were commissioned by the American Petroleum Institute.

John Barton, Deputy Director of the Buxton laboratory, says that so
far only one system has worked successfully in the test rig. The rig is
a mock vapour recovery pipe-line in which the flame arresters can be placed
at varying distances from the ignition site to allow testing at variable
flame speeds.

In the longer term, Barton hopes to be able to use computer simulation
to evaluate flame arresters. Suitable simulations are being developed jointly
with researchers at the Atomic Energy Authority’s site in Harwell, Oxfordshire.

When he quickly moved one pole of a bar magnet towards the coil, the
meter showed a brief pulse of current. When he pulled the magnet away, the
meter again showed current for a short time, but in the opposite direction.
When he held the magnet still, there was no current. By moving the magnet
rapidly to and fro in this way, you can produce an alternating current –
so called because it moves first in one direction and then in the other.
The speed at which you move the magnet back and forth (or rotate it) determines
the frequency of the alternations in direction.

This discovery was a great breakthrough in understanding the relationship
between the two forces. It led to many practical applications, not least
the generation of electricity, such as that supplied through the mains in
Britain. Before Faraday’s discovery, scientists used batteries as their
main source of electricity. Batteries produce a direct current, which flows
in only one direction.

When scientists first studied electric currents, they agreed that they
would consider current to flow from the positive terminal to the negative
terminal of a battery. Later, when they discovered electrons, they realised
that electrons flow from the negative terminal to the positive terminal.
Because like charges repel, this is the same as saying that electrons carry
a negative charge. It would be logical to get rid of the confusion by swapping
the labels on the two terminals, and calling the charge on the electron
positive, but the convention is too familiar.

Faraday showed that we can really consider magnetic forces and electrical
forces as two different aspects of the same thing.

What Maxwell did was to combine the laws of electricity and magnetism
into one set of mathematical equations. Although Faraday had shown that
a changing magnetic field causes an electric field, scientists then did
not understand what it was that related the electric and magnetic forces.
Nor did they understand exactly how the two forces influenced each other.

Maxwell found that he was forced to add an extra mathematical term to
make the laws of electricity and magnetism completely compatible. The improvement
meant that scientists soon discovered solutions to the equations that described
waves travelling forever through space. In effect, the varying electric
field produced a varying magnetic field, which in turn produced a varying
electric field, and so on as the wave went along. These solutions to the
equations contained a constant which could only be the speed at which the
waves move. That constant turned out to be the speed of light – the value
that the Danish astronomer Ole Romer had estimated as long ago as 1676.
Maxwell realised that his new equations were describing how light travels
through space. His new theory united light with electricity and magnetism.

Maxwell’s equations have remained a foundation of physics for more than
100 years, even though there has been a revolution in physics during this
time. Albert Einstein found that Newton’s laws of motion did not fit with
Maxwell’s equations. He modified Newton’s laws but kept Maxwell’s equations
intact when he developed his special theory of relativity. The fundamental
constant that is so important in Einstein’s theory (the speed of light,
c, aproximately 300 X 10**6 metres per second) emerges naturally from Maxwell’s
equations.

Electromagnetic waves can travel tremendous distances. They appear in
many different guises, as light, as radio and television broadcasts, as
radar, X-rays and cosmic rays.

Current only flows through a conductor if there is a pressure to push
the electrons along it. One way to think of this pressure is to imagine
what happens to a tank of water placed above a sink. If a pipe connects
the tank to the sink, the pull of gravity will force water in the tank to
move down the pipe. If the pipe is wide, there is little resistance, but
if the pipe is narrow, the flow of water will be reduced.

In electrical terms, a conductor takes the place of the pipe and the
‘pressure’ which pushes the current down the conductor is the voltage or
potential difference (pd) between its ends. Voltage is a measure of the
loss of electrical potential energy when one coulomb of charge flows down
the conductor. One volt is one joule per coulomb.

Electrical resistance to current flow is measured in ohms. A resistance
of 1 ohm will allow a current of 1 amp to flow for each volt of pd across
it.

The strength of a magnetic field is measured by its flux density. This
describes the density of lines of force passing through a loop in the field.
The SI unit of magnetic flux density is the tesla. One tesla is about a
hundred thousand times stronger than the magnetic field at the surface of
the Earth. The magnetic field near the poles of a toy horseshoe magnet is
about 0.5 tesla.

The faster the oscillation, the higher the frequency and the shorter
the wavelength.

Faraday’s discovery of electromagnetic induction, the process by which
magnets and coils produce currents, is used in electricity generators or
dynamos. A modern generator consists of a rotating magnet surrounding a
coil connected to an outside circuit. By rotating a magnetic field around
the coil of wire it is possible to produce continuous, but alternating,
currents. The speed at which the magnet is rotated determines the frequency
of the alternation. Older generators had a rotating coil within a fixed
magnet: this design meant that collecting the current from the coil was
not very efficient. In Britain, the frequency of mains electricity is 50
Hz, or 50 cycles per second.

Alternating currents are convenient because it is easy to change the
voltage and current values by means of a transformer. Transformers consist
of two coils of wire wound on an iron core. Any change in the current in
the first coil induces a voltage across the second. If the second coil has
a greater number of turns, the voltage across the second coil will be ‘stepped
up’, or made higher than the voltage across the first coil. The current
is reduced in proportion, and the power (given by voltage X current) remains
constant. If there are more turns in the first coil, the voltage is ‘stepped
down’. Step up and step down transformers are very important in large systems
for distributing electricity. Power plants step up the electricity they
generate to very high voltages. This reduces the current, which in turn
reduces the amount of power that the electricity loses as it is being transmitted.
When it arrives, the electricity is stepped down to the lower voltages we
use.

Electric motors are simply dynamos reversed in their functions: the
same principles govern their design as that of generators. They are central
to a wide range of traction and domestic devices. In 1821, Faraday made
the first electric system to produce continuous rotation. Engineers, though,
did not develop practical electric motors until the 1870s and 80s, when
they had advanced greatly their knowledge of generators.

At its simplest, the motor consists of an armature, a coil of wire of
many turns wrapped around an iron cylinder, freely suspended in a magnetic
field. The armature turns when a current is passed through its coil.

Virtually all domestic devices with an electric motor, including tape
decks and washing machines, use alternating current and convert, or ‘rectify’,
it to direct current. Most large motors, such as railways and metros use,
depend on direct current, but engineers are now beginning to adapt them
to alternating current.

On the higher frequency side, microwaves, which have frequencies in
the range 300 to 30,000 megahertz or MHz (1 MHz = 1 X 10**6 hertz) and wavelengths
between about 1 metre and 1 centimetre, can be used to heat food. They do
this by causing molecules within the food to vibrate.

Microwaves encourage water molecules in food to align themselves in
the direction of the electric field. In a microwave oven, the electric field
changes direction 2450 X 10**6 times per second. Each time the field changes
direction, the water molecules are flipped over. This makes them jostle
one another and so the temperature of the food rises. The process cooks
the food from the inside.