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How Light Bulbs Work
Before the invention of the light bulb, illuminating the world after the sunwent
down was a messy, arduous, hazardous task. It took a bunch
of candlesor torchesto fully light up a good-sized room, and oil lamps, while fairly
eective, tended to leave a residue of soot on anything in their general vicinity.
When the science of electricity really got going in the mid !""s, inventors
everywhere were clamoring to devise a practical, aordable electrical home lightingdevice. #nglishman $ir %oseph $wan and &merican 'homas #dison both got it right
around the same time (in !)! and !)*, respectively+, and within years,
millions of people around the world had installed electricallighting in their homes.
'he easy-to-use technology was such an improvement over the old ways that the
world never looked back.
'he amazing thing about this historical turn of events is that the light bulb itself
could hardly be simpler. 'he modern light bulb, which hasnt changed drastically
since #disons model, is made up of only a handful of parts. In this article, well see
how these parts come together to produce bright light for hours on end.
/ight Basics
/ightis a form of energy that can be released by an atom. It is made up of many
small particle-like packets that have energy and momentum but no mass. 'hese
particles, called light photons, are the most basic units of light. (0or more
information, see 1ow /ight Works.+
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&toms release light photons when their electrons become e2cited. If youve
read 1ow &toms Work, then you know that electrons are the negatively charged
particles that move around an atoms nucleus (which has a net positive charge+. &n
atoms electrons have dierent levels of energy, depending on several factors,
including their speed and distance from the nucleus. #lectrons of dierent energy
levels occupy dierent orbital3s. 4enerally speaking, electrons with greater energymove in orbitals farther away from the nucleus. When an atom gains or loses
energy, the change is e2pressed by the movement of electrons. When something
passes energy on to an atom, an electron may be temporarily boosted to a higher
orbital (farther away from the nucleus+. 'he electron only holds this position for a
tiny fraction of a second5 almost immediately, it is drawn back toward the nucleus,
to its original orbital. &s it returns to its original orbital, the electron releases the
e2tra energy in the form of a photon, in some cases a light photon.
'he wavelength of the emitted light (which determines its color+ depends on how
much energy is released, which depends on the particular position of the electron.
6onse7uently, dierent sorts of atoms will release dierent sorts of light photons. In
other words, the color of the light is determined by what kind of atom is e2cited.
'his is the basic mechanism at work in nearly all light sources. 'he main dierence
between these sources is the process of e2citing the atoms.
In the ne2t section well look at the dierent parts of a light bulb.
/ight Bulb $tructure
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/ight bulbs have a very simple structure. &t the base, they have two metal contacts,
which connect to the ends of an electricalcircuit. 'he metal contacts are attached
to two sti wires, which are attached to a thin metal 8lament. 'he 8lament sits in
the middle of the bulb, held up by a glass mount. 'he wires and the 8lament are
housed in a glass bulb, which is 8lled with an inert gas, such as argon.
When the bulb is hooked up to a power supply, an electric current 9ows from one
contact to the other, through the wires and the 8lament. #lectric current in a solid
conductor is the mass movement of free electrons (electrons that are not tightly
bound to an atom+ from a negatively charged area to a positively charged area.
&s the electrons zip along through the 8lament, they are constantly bumping into
the atoms that make up the 8lament. 'he energy of each impact vibrates an atom --
in other words, the current heats the atoms up. & thinner conductor heats up more
easily than a thicker conductor because it is more resistant to the movement of
electrons.
Bound electrons in the vibrating atoms may be boosted temporarily to a higher
energy level. When they fall back to their normal levels, the electrons release the
e2tra energy in the form of photons. :etal atoms release mostly infrared light
photons, which are invisible to the human eye. But if they are heated to a high
enough level -- around ;,""" degrees 0ahrenheit (,"" degrees 6+ in the case of a
light bulb -- they will emit a good deal of visible light.
'he 8lament in a light bulb is made of a long, incredibly thin length
of tungsten metal. In a typical
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when one of the chemicals has reached its ignition temperature. =n #arth,
combustion is usually a reaction between o2ygen in the atmosphere and some
heated material, but other combinations of chemicals will combust as well.
'he 8lament in a light bulb is housed in a sealed, o2ygen-free chamber to prevent
combustion. In the 8rst light bulbs, all the air was sucked out of the bulb to create anear vacuum -- an area with no matter in it. $ince there wasnt any gaseous matter
present (or hardly any+, the material could not combust.
'he problem with this approach was the evaporation of the tungsten atoms. &t such
e2treme temperatures, the occasional tungsten atom vibrates enough to detach
from the atoms around it and 9ies into the air. In a vacuum bulb, free tungsten
atoms shoot out in a straight line and collect on the inside of the glass. &s more and
more atoms evaporate, the 8lament starts to disintegrate, and the glass starts to
get darker. 'his reduces the life of the bulb considerably.
In a modern light bulb, inert gases, typically argon, greatly reduce this loss of
tungsten. When a tungsten atom evaporates, chances are it will collide with an
argon atom and bounce right back toward the 8lament, where it will re>oin the solid
structure. $ince inert gases normally dont react with other elements, there is no
chance of the elements combining in a combustion reaction.
6heap, eective and easy-to-use, the light bulb has proved a monstrous success. It
is still the most popular method of bringing light indoors and e2tending the day
after sundown. But by all indications, it will eventually give way to more advanced
technologies, because it isnt very e?cient.
Incandescent light bulbs give o most of their energy in the form of heat-carrying
infrared light photons -- only about " percent of the light produced is in the visible
spectrum. 'his wastes a lot of electricity. 6ool light sources, such as 9uorescent
lampsand /#@s, dont waste a lot of energy generating heat -- they give o mostly
visible light. 0or this reason, they are slowly edging out the old reliable light bulb.
How Fluorescent Lamps Work
Aou see 9uorescent lighting everywhere these days -- in o?ces, stores, warehouses,
street corners... Aoull even 8nd 9uorescent lamps in peoples homes. But even
though theyre all around us, these devices are a total mystery to most people. %ust
what is going on inside those white tubes
In this article, well 8nd out how 9uorescent lamps emit such a bright glow without
getting scalding hot like an ordinary light bulb. Well also 8nd out why 9uorescent
lamps are more e?cient than incandescent lighting, and see how
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Let There Be Light
'o understand 9uorescent lamps, it helps to know a little about light itself. /ight is a
form of energy that can be released by an atom. It is made up of many small
particle-like packets that have energy and momentum but no mass. 'hese particles,called light photons, are the most basic units of light. (0or more information,
see 1ow /ight Works.+
&toms release light photons when their electronsbecome e2cited. If youve
read 1ow &toms Work, then you know electrons are the negatively charged particles
that move around an atoms nucleus (which has a net positive charge+. &n atoms
electrons have dierent levels of energy, depending on several factors, including
their speed and distance from the nucleus. #lectrons of dierent energy levels
occupy dierent orbitals. 4enerally speaking, electrons with greater energy move
in orbital farther away from the nucleus.
When atom gains or losses energy, the change is e2pressed by the movement of
electrons. When something passes energy on to an atom -- heat, for e2ample -- an
electron may be temporarily boostedto a higher orbital (farther away from the
nucleus+. 'he electron only holds this position for a tiny fraction of a second5 almost
immediately, it is drawn back toward the nucleus, to its original orbital. &s it returns
to its original orbital, the electron releases the e2tra energy in the form of a photon,
in some cases a light photon.
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'he wavelengthof the emitted light depends on how much energy is released,
which depends on the particular position of the electron. 6onse7uently, dierent
sorts of atoms will release dierent sorts of light photons. In other words,
the colorof the light is determined by what kind of atom is e2cited.
'his is the basic mechanism at work in nearly all light sources. 'he main dierencebetween these sources is the process of e2citing the atoms. In an incandescent
light source, such as an ordinary light bulbor gas lamp, atoms are e2cited by
heat5 in a light stick, atoms are e2cited by a chemical reaction. 0luorescent lamps
have one of the most elaborate systems for e2citing atoms, as well see in the ne2t
section.
Down the Tubes
'he central element in a 9uorescent lamp is a sealed glass tube. 'he tube
contains a small bit of mercuryand an inert gas, typically argon, kept under very
low pressure. 'he tube also contains a phosphor powder, coated along the inside
of the glass. 'he tube has twoelectrodes, one at each end, which are wired to an
electrical circuit. 'he electrical circuit, which well e2amine later, is hooked up to an
alternating current (&6+ supply.
When you turn the lamp on, the current 9ows through the electrical circuit to the
electrodes. 'here is a considerable voltage across the electrodes, so electrons will
migrate through the gas from one end of the tube to the other. 'his energy changes
some of the mercuryin the tube from a li7uid to a gas. &s electrons and charged
atoms move through the tube, some of them will collidewith the gaseous mercuryatoms. 'hese collisions e2cite the atoms, bumping electrons up to higher energy
levels. When the electrons return to their original energy level, they release light
photons.
&s we saw in the last section, the wavelength of a photon is determined by the
particular electron arrangement in the atom. 'he electrons in mercury atoms are
arranged in such a way that they mostly release light photons in
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the ultravioletwavelength range. =ur eyes dont register ultraviolet photons, so
this sort of light needs to be converted into visible light to illuminate the lamp.
'his is where the tubes phosphor powder coating comes in. Phosphorsare
substances that give o light when they are e2posed to light. When a photon hits a
phosphor atom, one of the phosphors electrons >umps to a higher energy level andthe atom heats up. When the electron falls back to its normal level, it releases
energy in the form of another photon. 'his photon has less energy than the original
photon, because some energy was lost as heat. In a 9uorescent lamp, the emitted
light is in the visible spectrum -- the phosphor gives o white lightwe can see.
:anufacturers can vary the color of the light by using dierent combinations of
phosphors.
6onventional incandescent light bulbs also emit a good bit of ultraviolet light, but
they do not convert any of it to visible light. 6onse7uently, a lot of the energy used
to power an incandescent lamp is wasted. & 9uorescent lamp puts this invisible light
to work, and so is more efcient. Incandescent lamps also lose more energythrough heat emission than do 9uorescent lamps. =verall, a typical 9uorescent lamp
is four to si2 times more e?cient than an incandescent lamp. Ceople generally use
incandescent lights in the home, however, since they emit a DwarmerD light -- a light
with more red and less blue.
&s weve seen, the entire 9uorescent lamp system depends on an electrical current
9owing through the gas in the glass tube. In the ne2t section, well see what a
9uorescent lamp needs to do to establish this current.
ooking with !as
In the last section, we saw that mercury atoms in a 9uorescent lamps glass tube are
e2cited by electrons 9owing in an electrical current. 'his electrical current is
something like the current in an ordinary wire, but it passes through gas instead of
through a solid. !as conductorsdier from solid conductors in a number of ways.
In a solid conductor, electrical charge is carried by free electrons >umping from atom
to atom, from a negatively-charged area to a positively-charged area. &s weve
seen, electrons always have a negative charge, which means they are always drawn
toward positive charges. In a gas, electrical charge is carried by "ree
electronsmoving independently of atoms. 6urrent is also carried by ions, atoms
that have an electrical charge because they have lost or gained an electron. /ikeelectrons, ions are drawn to oppositely charged areas.
'o send a current through gas in a tube, then, a 9uorescent light needs to have two
thingsE
. Free electrons and ions
. # di$erence in charge between the two ends o" the tube (a voltage+
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4enerally, there are few ions and free electrons in a gas, because all of the atoms
naturally maintain a neutral charge. 6onse7uently, it is di?cult to conduct an
electrical current through most gases. When you turn on a 9uorescent lamp, the
8rst thing it needs to do is introduce many new "ree electronsfrom both
electrodes.
%tart it &p
'he classic 9uorescent lamp design, which has fallen mostly by the wayside, used a
special starter switch mechanism to light up the tube. Aou can see how this system
works in the diagram below.
When the lamp 8rst turns on, the path of least resistance is through the bypass
circuit, and across thestarter switch. In this circuit, the current passes through
the electrodes on both ends of the tube. 'hese electrodes are simple 'laments,
like you would 8nd in an incandescent light bulb. When the current runs through the
bypass circuit, electricity heats up the 8laments. 'his boils o electrons from the
metal surface, sending them into the gas tube, ionizing the gas.
&t the same time, the electrical current sets o an interesting se7uence of events in
the starter switch. 'he conventional starter switch is a small discharge bulb,
containing neon or some other gas. 'he bulb has two electrodes positioned right
ne2t to each other. When electricity is initially passed through the bypass circuit,
an electrical arc(essentially, a 9ow of charged particles+ >umps between these
electrodes to make a connection. 'his arc lights the bulb in the same way a larger
arc lights a 9uorescent bulb.
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=ne of the electrodes is abimetallic stripthat bends when it is heated. 'he small
amount of heat from the lit bulb bends the bimetallic strip so it makes contact with
the other electrode. With the two electrodes touching each other, the current
doesnt need to >ump as an arc anymore. 6onse7uently, there are no chargedparticles 9owing through the gas, and the light goes out. Without the heat from the
light, the bimetallic strip cools, bending away from the other electrode. 'his opens
the circuit.
(nside the casing o" a conventional )uorescent starter there is a small gas
discharge lamp*
By the time this happens, the 8laments have already ionized the gas in the
9uorescent tube, creating an electrically conductive medium. 'he tube >ust needs a
voltage kick across the electrodes to establish an electrical arc. 'his kick is provided
by the lamp3sballast, a special sort of transformer wired into the circuit.
When the current 9ows through the bypass circuit, it establishes a magnetic
'eldin part of the ballast. 'his magnetic 8eld is maintained by the 9owing current.
When the starter switch is opened, the current is brie9y cut o from the ballast. 'he
magnetic 8eld collapses, which creates a sudden >ump in current -- the ballast
releases its stored energy.
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+apid start and starter switch )uorescent bulbs have two pins that slide
against two contact points in an electrical circuit*
Light +ight #way
'oday, the most popular 9uorescent lamp design is therapid startlamp. 'his
design works on the same basic principle as the traditional starter lamp, but it
doesnt have a starter switch. Instead, the lamps ballastconstantly channelscurrent through both electrodes. 'his current 9ow is con8gured so that there is a
charge dierence between the two electrodes, establishing a voltage across the
tube.
When the 9uorescent light is turned on, both electrode 8laments heat up very
7uickly, boiling o electrons, which ionize the gas in the tube. =nce the gas is
ionized, the voltage dierence between the electrodes establishes an electrical arc.
'he 9owing charged particles (red+ e2cite the mercury atoms (silver+, triggering the
illumination process.
&n alternative method, used in instant,start9uorescent lamps, is to apply a veryhigh initial voltage to the electrodes. 'his high voltage creates a corona discharge.
#ssentially, an e2cess of electrons on the electrode surface forces some electrons
into the gas. 'hese free electrons ionize the gas, and almost instantly the voltage
dierence between the electrodes establishes an electrical arc.
Fo matter how the starting mechanism is con8gured, the end result is the sameE a
9ow of electrical current through an ionized gas. 'his sort of gas dischargehas a
peculiar and problematic 7ualityE If the current isnt carefully controlled, it will
continually increase, and possibly e2plode the light 82ture. In the ne2t section, well
8nd out why this is and see how a 9uorescent lamp keeps things running smoothly.
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The ballast- starter switch and )uorescent bulb are all wired together in a
simple circuit*
'his surgein current helps build the initial voltage needed to establish the
electrical arc through the gas. Instead of 9owing through the bypass circuit and
>umping across the gap in the starter switch, the electrical current 9ows through the
tube. 'he free electrons collide with the atoms, knocking loose other electrons,
which create ions. 'he result is plasma, a gas composed largely of ions and free
electrons, all moving freely. 'his creates a path for an electrical current.
'he impact of 9ying electrons keeps the two 8laments warm, so they continue to
emit new electrons into the plasma. &s long as there is &6 current, and the
8laments arent worn out, current will continue to 9ow through the tube.
'he problem with this sort of lamp is it takes a few seconds for it to light up. 'hese
days, most 9uorescent lamps are designed to light up almost instantly. In the ne2t
section, well see how these modern designs work.
Ballast Balance
We saw in the last section that gases dont conduct electricity in the same way as
solids. =ne ma>or dierence between solids and gases is their electrical
resistance(the opposition to 9owing electricity+. In a solid metal conductor such as
a wire, resistance is a constant at any given temperature, controlled by the size of
the conductor and the nature of the material.
In a gas discharge, such as a 9uorescent lamp, current causes resistance to
decrease. 'his is because as more electrons and ions 9ow through a particular area,
they bump into more atoms, which frees up electrons, creating more charged
particles. In this way, current will climb on its own in a gas discharge, as long as
there is ade7uate voltage (and household &6 current has a lot of voltage+. If thecurrent in a 9uorescent light isnt controlled, it can blow outthe various electrical
components.
0luorescent lamps ballastworks to control this. 'he simplest sort of ballast,
generally referred to as magneticballast, works something like an inductor. & basic
inductor consists of a coil of wire in a circuit, which may be wound around a piece of
metal. If youve read 1ow #lectromagnets Work, you know that when you send
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electrical current through a wire, it generates a magnetic 8eld. Cositioning the wire
in concentric loops ampli8es this 8eld.
'his sort of 8eld aects not only ob>ects around the loop, but also the loop itself.
Increasing the current in the loop increases the magnetic 8eld, which applies a
voltage opposite the 9ow of current in the wire. In short, a coiled length of wire in acircuit (an inductor+ opposes change in the current 9owing through it.
'he trans"ormer elementsin a magnetic ballast use this principle to regulate the
current in a 9uorescent lamp.
Ballast can only slow down changes in current -- it cant stop them. But the
alternating current powering a 9uorescent light is constantly reversingitself, so
the ballast only has to inhibit increasing current in a particular direction for a short
amount of time. 6heck out this sitefor more information on this process.
:agnetic ballasts modulate electrical current at a relatively low cycle rate, which
can cause a noticeable 9icker. :agnetic ballasts may also vibrate at a low
fre7uency. 'his is the source of the audible humming sound people associate with
9uorescent lamps.
:odern ballast designs use advanced electronics to more precisely regulate the
current 9owing through the electrical circuit. $ince they use a higher cycle rate, you
dont generally notice a 9icker or humming noise coming from electronic ballast.
@ierent lamps re7uire specialized ballasts designed to maintain the speci8c
voltage and current levels needed for varying tube designs.
0luorescent lamps come in all shapes and sizes, but they all work on the same basic
principleE &n electric current stimulates mercury atoms, which causes them to
release ultraviolet photons. 'hese photons in turn stimulate a phosphor, which
emits visible light photons. &t the most basic level, thats all there is to itG
How +elays Work
& relay is a simple electromechanical switchmade up of an electromagnet and a
set of contacts. Helays are found hidden in all sorts of devices. In fact, some of the
8rst computers ever built used relays to implement Boolean gates.
In this article, we will look at how relays work and a few of their applications
+elay onstruction
Helays are amazingly simple devices. 'here are four parts in every relayE
.lectromagnet
#rmaturethat can be attracted by the electromagnet
%pring
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%et o" electrical contacts
'he following 8gure shows these four parts in actionE
In this 8gure, you can see that a relay consists of two separate and completely
independent circuits. 'he 8rst is at the bottom and drives the electromagnet. In
this circuit, a switch is controlling power to the electromagnet. When the switch is
on, the electromagnet is on, and it attracts the armature (blue+. 'he armature is
acting as a switch in the second circuit. When the electromagnet is energized, the
armature completes the second circuit and the light is on. When the electromagnet
is not energized, the spring pulls the armature away and the circuit is not complete.
In that case, the light is dark.
When you purchase relays, you generally have control over several variablesE
'he voltage and current that is needed to activate the armature
'he ma2imum voltage and current that can run through the armature and thearmature contacts
'he number of armatures (generally one or two+
'he number of contacts for the armature (generally one or two -- the relay
shown here has two, one of which is unused+
Whether the contact (if only one contact is provided+ is normally open (/0+
or normally closed (/+
+elay #pplications
In general, the point of a relay is to use a small amount of power in
theelectromagnet-- coming, say, from a small dashboard switch or a low-power
electronic circuit -- to move an armature that is able to switch a much larger amount
of power. 0or e2ample, you might want the electromagnet to energize using volts
and " milliamps (" mill watts+, while the armature can support " &6 at
amps (;" watts+.
Helays are 7uite common in home appliances where there is an electronic control
turning on something like a motor or a light. 'hey are also common in cars, where
the supply voltage means that >ust about everything needs a large amount of
current. In later model cars, manufacturers have started combining relay panels into
the fuse bo2 to make maintenance easier. 0or e2ample, the si2 gray bo2es in this
photo of a 0ord Windstar fuse bo2 are all relaysE
In places where a large amount of power needs to be switched, relays are
often cascaded. In this case, a small relay switches the power needed to drive a
much larger relay, and that second relay switches the power to drive the load.
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