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Chapter 3
Putting electricity to use.
Electric motors and telegraph were the first wide spread use of electricity. Faraday
produced a crude electric motor (1821) before William Sturgeon demonstrated the
first efficient electromagnet (1825) and well before Samuel Morse developed the first
telegraph (1837).
For technical reasons that will become apparent we will look first at the
electromagnet, then the telegraph and finally the electric motor.
Each of these inventions had far reaching effects on the development of electricity
and electronics. Electromagnets are still used today in solenoid switches and the like.
From early work with these devices sprang the idea for telegraphs. This in turn led
directly to telephones and more indirectly to radio. Electric motors have become so
ubiquitous that you probably don’t realise how many of them surround you. Washing
machines may immediately come to mind but what about your printer or CD player or
microwave oven or even your mobile phone (an electric motor makes them vibrate
when a call comes in). Electrical pioneers like Faraday and Oersted probably had little
idea how much their efforts would shape the future.
Electromagnets.
These devices are generally quite simple in
operation. If current is supplied to a wire coiled
about a bar of iron a magnetic field will be
generated around this device. We made a crude
electromagnet using a bolt and enamelled copper
wire. This wire has a flexible coating of insulation
around it and is used to make generators, motors,
solenoids and many other devices. The coating can
easily be scraped away exposing copper wire to
form electrical connections. You can buy a roll of
enamelled copper wire for a few dollars from
electronic hobby stores like Radio Shack. Thin wire
with plastic insulation will also work in this
instance. Uninsulated wire is no good because as
soon as conductive coils of metal touch electricity
will jump from one coil to another instead of going right the way around in a loop.
Electricity will always take the shortest path
available. We used about 100 turns of wire around
the bolt and a 9-volt battery to power the
electromagnet. If you decide to make an
electromagnet like the one pictured it is important
to only connect it to a small battery (9 volts is about
right). Too big a battery (e.g. a car battery) can
supply far too much current causing the wire to heat
up rapidly and quite probably melt. Please don’t
even think of connecting your electromagnet (or
any other DC device) to mains electricity, as this
is very dangerous.
The first Telegraph.
Our electromagnet simply
picked up a few paper clips but
more efficient ones can do all
sorts of useful things. They can
operate a valve like in garden
reticulation systems or can
open (or close) a switch like the
relays in your car. They can
ring a bell like the one
illustrated. When the switch is
closed current flows through
the electromagnet’s coils. A
magnetic field is generated
pulling the iron armature
towards the electromagnet. A hammer attached to the armature will then strike the
bell. This was the very system used to demonstrate the first practical telegraph. The
only real difference being that battery and switch was approximately one mile away
from the electromagnet and bell. Samuel Morse then set about developing a code that
could easily be deciphered by an operator listening to the bell. Hence Morse code was
born. The bell was later replaced by a pen that wrote upon a length of paper drawn by
an electric motor. Operators quickly learnt to recognise code from the tapping noises
made by the pen. This led to a simplified system whereby the pen and paper were
deleted leaving just an armature to tap out its noisy code.
Electric motors.
Lets begin by imagining an
electromagnet mounted on a
spindle, this arrangement is
known as an armature. A
permanent magnet is
mounted directly below the
armature. We won’t worry
about how the electromagnet
is powered at this stage.
While there is no current
supplied to the armature it
will rest in whatever position
it was left in. Once current is
supplied a magnetic field is
generated which will be
attracted to the permanent magnet. The armature will turn on its spindle to align with
the magnet. Once aligned the attraction between armature and magnet will prevent
any further rotation. If the armature is to keep rotating then current will need to be
switched off just as the armature and permanent become aligned. By switching the
current on and off at just the right time our armature will remain spinning. A device
called a commutator achieves this effect.
Simple motor.
We built a simple electric motor from enamelled wire and
a permanent magnet (see photo). The wire was wound
twenty times around a broom handle to form a coil
leaving a long strand of wire at each end. One strand was
stripped of insulating enamel (an old hacksaw blade does
a good job) while the other only had enamel removed
from one side along its length. Half of the strand was
exposed and half insulated. The coil was then mounted in
a crude wooden cradle. Two lengths of tie wire (any stiff
uninsulated wire will do) were then screwed to the base
and bent so that they rubbed against the outstretched strands of enamelled wire. These
tie wires now form what is known as brushes whose purpose is to supply electric
current to the coil. As one of the coils connecting strands has only had half its
insulation stripped away electric current only flows through the coil during half of its
rotation. Once the coil starts to spin an electric current is continuously switched on
and off causing a magnetic field to also be created then dissipated around the coil.
This field is attracted by the permanent magnet keeping our simple motor spinning
happily.
Motor with commutator.
The second motor we built had four electromagnets
mounted on a wooden block. These consisted of one
hundred turns of enamelled wire wrapped around
steel wood screws. Long strands of enamelled wire
were left on each winding (see picture). Two nails
were inserted into the block to form a spindle. A short piece of wooden dowel was
mounted on one of the nails. A single strand from each of the coils was stripped of
enamel and attached to one of the nails. The four remaining strands were also stripped
of enamel and laid flat upon the wooden dowel. The armature was then mounted in a
wooden cradle and contact wires
(brushes) mounted on the base just
like the simple motor’s brushes were
mounted. One brush contacted the
wires mounted on the wooden dowel
while another was in contact with a
nail. We’ve drawn this armature with
just one winding to show how
current flows through the armature.
Starting at the left hand contact wire
current flows up to and through the
left hand nail, through windings (coil
of wire) and onto wire attached to
wooden dowel. The wooden dowel
and wires are called a commutator, its purpose is to connect and disconnect coils of an
electromagnet at just the right time to keep the armature rotating. From here current
will travel down the right hand contact wire and back to its source (e.g. battery). If a
permanent magnet is strategically placed the coils magnetic field will be drawn
towards it. Here are some small photo’s of the motor described above.
The commutator in our motor
switches each coil on then off
again as coil and magnet become
aligned. By doing so the motors
armature will keep spinning as
long as sufficient current is
supplied to the contact wires.
A word of caution to those who wish to build this motor. A 12 volt battery is
required and the motor windings can get very hot particularly if the armature is not
turning. Leaving a battery connected to motor for too long may cause it to catch
fire.
Dynamo’s reviseted
If we were to replace the battery running our motor with a volt meter then spun the
armature by hand we would see a voltage produced. Most permanent magnet type
electric motors will act as generators when spun by hand or driven by another source.
When we looked at a simple dynamo in chapter 2 we found that it produced
alternating current. That is to say current flowed in one direction as the magnet and
coil approached each other and in the opposite direction as magnet and coil moved
apart. The early developers of electric devices had difficulty useing alternating current
(AC) so a means of switching off current flow as magnet passes coil needed to be
found. It was soon found that a commutator can do just this (see illustration) so that a
source of direct current (DC) could be mechanically generated. The current wasn’t
smoothe like that of a battery (it pulsated with each pass between coil and magnet) but
at least it didn’t change direction.
The only difference between our motor and generator is that the motor was connected
to a battery and generator connected to a volt meter. Since the development of small,
cheap and reliable rectifiers DC (direct current) generators are gradually becoming
obsolete in favour of more efficient alternators. Where DC is required we use various
forms of rectifier to convert AC to DC power supply. We will be looking at how
rectifiers work in chapter 5. Most electricity is produced by alternators these days, our
domestic mains power is AC (alternating current) and home appliences are especially
designed to work with AC. Up until the end of 1960’s car electrical systems were
powered by a DC generator while the engine was running. Once high current solid
state rectifiers became available automotive AC alternators quickly replaced the less
efficient DC generators.
Telephones.
Many people know that Alexander
Graham Bell was the first to patent a
telephone in 1876. I wonder how many
know that he didn’t set out to develop a
telephone. Mr. Bell was actually trying
to increase the capacity of telegraph
systems by using oscillating (vibrating)
electrical signals of various frequencies.
He set up a mini telegraph system
between two rooms to test his ideas
when he realised that his ‘telegraph’ was
transmitting the sounds of a clock ticking
away in the other room. Thus the
telephone was born all be it somewhat
serendipitously. He was also fortunate to be credited with the telephone’s invention as
his patent application was filed only hours before a rival applied to patent his system.
We’ve allready looked at the electrical principles that led to the development of
telephones. If a magnet is passed by a coil of wire then an electric current becomes
available in that wire. We call this process magnetic induction and say that the magnet
induces an electrical current in the coil of wire. Lets mount a small magnet inside a
coil of wire and then attach that magnet to a flexible diaphragm. If the diaphragm
vibrates then the magnet will vibrate with it. As the magnet moves within the coil of
wire so a small electrical current will be produced (induced). This current will change
direction as the magnet changes direction and will increase in intensity as the magnets
movements also increase. The electrical current will in many ways mirror the
diaphragm’s movements. If sound waves cause the diaphragm to vibrate then the
magnet will induce an electrical signal into the coil that mirrors the sound . We have
just described the workings of a crude dynamic microphone.
Electrical signals can also be turned back into sound by way of a speaker. These work
in the opposite way to a microphone. Mechanically they are very similar in that a
magnet is attached to a diaphragm (a much larger one known as a speaker cone) and a
coil of wire surrounds the magnet. Those of you with knowledge of speakers will know
that they are actually arranged differently though the principle is exactly the same.
When the coil has an electrical signal applied it pulls on the magnet causing a
vibration to occur in the speaker cone. If we attach the coils of a microphone to a
distant speaker’s coil by way of wires then we have constructed a simple telephone.
We can follow the sequence in our illustration. Sound from the violin makes the
diaphragm vibrate. The magnet being attached will vibrate with the diaphragm
inducing an electrical signal into the coil that surrounds it. This signal will travel
down the wires into the speakers coil thus tugging on a magnet attached to the speaker
cone causing it to vibrate just as the microphone’s diaphragm did. Our violinist’s
music will then be audible from the speaker.
Speakers can be used as microphones though microphones don’t make very good
speakers (their diaphragm is too small for one thing).
Modern telephones are of course more sophisticated than this, they have a dialler, an
amplifier, a special coil to stop your own voice being audible in the earpiece and
plenty of other electronic tricks to make them as functional as they are. The basic
principle of turning a sound into an electrical signal and back into a sound at the other
end is still the same though.
Electric lighting.
Early reserchers into electrical devices soon found that passing too much electrical
current down a wire will cause that wire to get hot and if enough current is available
the wire will melt. It didn’t take long for them to realise that if they could get a wire to
glow bright enough then a source of electric lighting would be possible. There were
obstacles to over come though. Firstly there needed to be found a material that would
conduct electricity and be able to glow bright enough without melting. There was also
the problem of oxidation, suitable materials that got to white heat invariably burned
away quickly.
No one realy agrees as to who invented the electric light, Humphrey Davey (UK)
demonstrated both incandescent and arc lights around 1800 to 1804. Most early
electric lights were arc lamps which produced light by creating a spark between two
carbon rods. These lights were inefficient (used lots of electricity), were too bright for
many applications and tended to flicker unevenly.
Incandescent lamps (with glowing wires or filaments) were attempted by many
reserchers from 1802 onwards but it wasn’t untill between 1877 and 1878 that Joseph
Swan (UK) and Thomas Alva Edison (USA) both pioneered the use of durable carbon
filament incandescent lights.
The first successful light globes consisted of a bulbous tube of glass within which was
mounted two wires. Between the wires there hung a thin filament of carbonised
cotton. The glass tube was evacuated of air and sealed with the two wires ends
protruding outside. Electrical current was then fed to the two wires causing the
fillament to glow white hot.
About 1910 carbon filaments
were replaced by tungsten ones
and light bulbs were filled with
the inert gas argon instead of
being evacuated. The modern
electric light had arrived and has
changed little since.
During the early proliferation of
electric lighting a fierce debate
errupted as to whether they
should be powered by AC or DC
electric current. Edison stoically
led the campaign for DC but lost
out to his former emploee and
fierce rival Nicola Tesla for a
simple technical reason. The only
way to efficiently send electricity
down very long wires is to do so at high voltages. AC current can be raised or lowered
in voltage by devices called transformers alowing huge voltages to be sent down
transmission lines yet much safer voltages alowed into homes etc. Transformers don’t
work with DC current so Edison’s system was doomed from the start.
Transformers
Lucien Gaulard and John Gibbs demonstrated a transformer in London during 1881
although it was Nicola Tesla who developed the first efficient transformers a few
years later.
If two coils of wire are wrapped
arround the same iron bar and
one of those coils is supplied
with AC voltage then another
voltage will be created in the
second coil. As voltage
increases in coil 1 then the iron
bar will become an
electromagnet whose strength
and polarity pulsates along with
the input voltage. This
pulsating magnetic
field then creates a
voltage in coil 2. The
correct term for this
process is induction,
we say that a voltage
is induced in coil 2.
When both coils have
the same number of
turns then the votage
induced in coil 2
would be the same as
that supplied to coil 1.
Should coil 2 have twice as many turns as coil 1 then the voltage induced is also
doubled. Of course this doubling of voltage does not increase the total amount of
power available, you can’t create energy from nothing, when voltage is doubled
current is at least halved. You may recall the formula we presented in chapter 2;
Power(in Watts) = Amps x Volts. Power availbale at the transformers secondary
cannot exceed that supplied to its primary windings. The ratio of turns between coil 1
(primary coil) and coil two (secondary coil) will be the same as the ratio of voltage
between supply and output of transformer.. Differences in current between primary
and secondary also follow the same ratio but is inverted, i.e. if voltage is trebled then
output current becomes one third of input current. If coil 2 has less turns than coil 1
then voltage will be reduced and current increased
Although modern transformers are very
efficient they do lose a little of the power
supplied to the primary coil. Manufacturers
allow for this and add a few turns to one or
other of the coils to ensure the correct
voltage is available at the secondary or
output coil.
Transformers that are used to increase
voltage are called ‘step up transformers’ and
those used to reduce voltage are called ‘step
down transformers’.
Radio
The invention of radio owes much to the development of
transformers, telegraph and telephones. The first radio
transmitters were called radiotelegraphy transmitters and
shared many of telegraph’s components. The story of radio’s
genesis is both convoluted and controversial with a number
of people making crucial contributions.
In 1864 a brilliant Scottish mathematician called James
Clerk Maxwell predicted the existence and nature of radio
waves. His work was largely missunderstood so it wasn’t untill about three decades
later that the first radio’s were developed.
Even to this day there is considerable debate as to who actually invented radio.
Guglielmo Marconi is generally credited with radio’s invention although in 1933 a
US supreme court decision revoked Marconi’s patents in favour of Nikola Tesla.
Despite this there is no doubt that Marconi achieved a number of milestones in the
development of radio including the first trans atlantic radio message.
The German physicist Heinrich Hertz could also be credited with radio’s invention
and was the first to actually demonstrate the existence of radio waves in 1893. Around
about two years later Marconi unvieled a practical radiotelegraph setting in motion a
revolution that led to radio, radar, television, mobile phones and many other icons of
our technological age.
Early radio transmitters were rudimentary devices that created a spark between two
small metal spheres one of which was connected to a length of wire called an antenna.
Looking at the illustration we can follow the sequence of events.
A telegraphers switch briefly supplies current to the primary windings of a
transformer. The sudden surge in voltage is multiplied in the transformers secondary
winding causing a strong spark to jump between two closely mounted metal spheres.
A long piece of wire called an antenna would then pulse with electric charge and in
doing so emmit radio waves.
Recievers at this time employed an even cruder means of detecting radio waves. It
was known that dust or iron filings will tend to clump together (cohere) in the
presence of radio waves. Iron filings are normally poor conductors of electricity but
when they clump together or cohere they conduct electricty readily. The first radio
recievers were based on the coherer, a glass tube containing iron filings and sealed by
a metal cap at each end. Refering to the diagram, a battery is connected to one side of
the coherer while the other side is connected to an electromagnet. The electromagnet
in turn is also connected to the battery. When no radio waves are present very little
current passes through the coherer so the electromagnet does not attract the armature.
As soon as a burst of radio waves passed over the coherer its iron filings would cohere
allowing current to flow thus activating the electromagnet. One giant shortfall of a
coherer was that once the iron filings had clumped together the tube had be tapped to
dispurse them again hence this system was only good for the transmission of morse
code.
Coherers formed the basis of radio recievers for about the first ten years of their
production limiting the usefullness of this technology. It wasn’t untill the invention of
tuned oscillating circuits and electronic components like vacuum tubes that truly
modern radio was born.
We will take another look at radio in chapter 5.