How a Dual-Coil Magnet is Made
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Choosing a Core for the Dual-Coil Magnet
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A dual-coil magnet or bifilar coil magnet, first created and patented by Tesla, is used most often as a resistor in electronic appliances, a device created to oppose electric currents. Like other simple electromagnetic devices, a bifilar coil magnet is created with a specific type of wire, a core which the wire is wrapped around and an electrical charge that creates the magnetic field--although with dual-coil magnets, the purpose is often to dampen or dissipate the magnet field and lower inductance for AC electrical systems.
The core of the bifilar system is chosen for its low inductance and capabilities of taking on and losing a magnetic field. For this reason, air itself is often chosen as the primary core and was used by Tesla when he created the dual core design. Creators may also choose a core made from ferrite or some other type of material. These metal cores are mass produced in factories to certain specifications and may be "I" shaped in the form of a simple rod or shaped like a "U" or a "C", while more complicated cores are known as "E", "EFD", "ER" and "EP". While any of these cores may used, air is of course the least expensive, while for simply scientific experiments even a pair of nails will work. The advanced cores are covered with a laminate to reduce fluctuations.
Choosing Wire and a Purpose
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Once bifilar coil magnets became popular in many electronics, specialized bifilar wire was created for various electromagnetic uses. This wire can be wound around the core to create the electromagnet, but depending on which type of dual-coil magnet you desire you can personally wind the wire in several different ways. Coils in which the two wires are wound in parallel circles, so that the current flows in the same direction through both wires, will naturally magnify the magnetic field, creating a stronger, multiplied field.
If a resistor is desired, the two wires are wound so that the current flows in opposite directions around the core. The fields will then cancel each other out, and if the electromagnet is made correctly, the net magnetic field will be zero or close to it. If the coil is going to be used in a relay situation where the current is expected to be interrupted, then the wires can be electrically isolated and split into primary and secondary uses so that extra energy is diverted into heat by the secondary wire, and the primary wire can still function without damaging sensitive electronics part further down the system.
These wires are usually made of copper or another inexpensive, conductive material, and are mass-produced to various specifications. Few bifilar coil magnets are still hand made, and the wire is often sent to other factories, where automated assemblies wind them correctly to create the various electromagnet assemblies.
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Applying the Electromagnet
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The last step to creating an bifilar coil magnet is to place it in the system for which it is meant. In simple experiments, this can be as easy as attaching the wires to a battery or other power source. In electronic devices, the resistor is integrated into the circuit at an optimal place to control the current. The wires is such resistor modules may be made of different materials, such as nickel or chrome.
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Resources
- Photo Credit Tesla's Bifilar Coil Design, courtesy the Modern Energy Research Library.
