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| Rev. Nicholas Callan Public domain image |
Did you know that the induction coil is an Irish invention by Reverend Nicholas Callan (1799 - 1864), a physicist and Catholic priest at Maynooth University?
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.
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| Schematic of a transformer BillC at the English-language Wikipedia, CC BY-SA 3.0, via Wikimedia Commons |
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, such as a loop of wire, a voltage can be induced in the wire. If the circuit is closed, this voltage can drive an electric current. It is the change in magnetic flux through the circuit that produces the induced voltage, rather than simply the movement of the magnet itself. The magnetic field can remain constant in magnitude, provided the magnetic flux through the circuit changes. The magnitude of the induced voltage is proportional to the rate of change of magnetic flux linkage. In other words, the faster the magnetic flux linkage changes, the greater the induced 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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| An induction coil Hannes Grobe, CC BY 3.0, via Wikimedia Commons |
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.