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| Rev. Nicholas Callan. Public domain image via Wikimedia Commons: https://en.wikipedia.org/wiki/Nicholas_Callan... |
The device is a type of
transformer, commonly used in vehicles in the form of an ignition coil
for generating the tens of thousands of volts necessary to create a
spark at the plugs. Induction coils with an output of hundreds of
thousands of volts were used for early spark-gap transmitters. Connected
to a tuned circuit and antenna, they could transmit radio waves at a
certain frequency. The tuned circuit is the electrical analog of a bell.
A spark discharge "rang" the bell by setting off electrical
oscillations.. These transmitters would have been used for
intercontinental and ship-to-shore communications in the era of the
Titanic.
Electromagnetic Induction
Electromagnetic
induction is a phenomenon discovered by the English scientist Michael
Faraday in 1831. If you move a magnet close to an electrical conductor,
e.g. a looped piece of wire, the field of the magnet induces an electric
current in the wire. It's the change in magnitude of the magnetic field
that creates the current, rather than the fact that the magnet is
moving. If the field is constant in magnitude, no current is induced.
The magnitude of the voltage that creates the current is proportional to
the rate of change of magnetic flux, in other words, the faster the
magnetic field changes in size, the greater the voltage.
How Do Transformers Work?
All transformers and electrical generators work on the principle of electromagnetic induction.
The
transformers such as those used in older power adapters and corded
electronic equipment have a laminated core, made of a stack of
wafer-thin, soft iron sections, insulated from each other. A primary
coil wound around the core (hundreds of turns of wire) generates a
fluctuating current when connected to an AC mains source. This in turn
creates a fluctuating magnetic field and that field then induces an
electric current in a secondary winding. A transformer simply increases
voltage, or decreases it, like the room sized transformers we have at
the sub-station in Kilcullen. The ratio of the number of turns of wire
on the primary coil to those on the secondary coil, known as the turns
ratio, determines the factor by which the output is increased or
decreased. So if the input coil has 1000 turns and the secondary coil
has 100 turns, that's a turns ratio of 10 and voltage is reduced by a
factor of 10
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| Schematic of a transformer. Image author BillC at the English-language Wikipedia, CC BY-SA 3.0 |
What are Induction Coils?
Instead
of being fed by AC, an induction coil works on DC. It has a primary
coil with hundreds of turns of wire and a secondary with thousands of
turns. The coil when energised also acts like an electromagnet. This
isn't the case with a standard transformer because the iron core is in
the form of a closed loop, with no external field. The DC source, e.g. a
battery, is connected to the coil via a spring switch, called an
interrupter, that can be opened and closed by the force of the
electromagnet (the same mechanism is used on old doorbells). On
connection of the supply, with the switch closed, the magnetic field in
the core grows until the electromagnet suddenly opens the switch,
disconnecting the input. The magnetic field rapidly collapses and it's
this rapid collapse and change in field that induces a huge voltage in
the secondary, much greater than the turns ratio could produce with an
AC source. When current is disconnected by the switch, the force of the
electromagnet drops to zero, causing the spring to close the switch
again, reconnecting the circuit and the cycle repeats indefinitely,
generating high voltage pulses at the rate of hundreds per second.
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| An induction coil. Image courtesy Hannes Grobe, CC BY 3.0 via Wikimedia Commons |
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