thumb|Non-inductive bifilar winding

thumb|280px|[[Nikola Tesla's flat inductive bifilar coil]]

A bifilar coil is an electromagnetic coil that contains two closely spaced, parallel windings. In electrical engineering, the word bifilar describes wire which is made of two filaments or strands. It is commonly used to denote special types of winding wire for transformers. Wire can be purchased in bifilar form, usually as different colored enameled wire bonded together. For three strands, the term trifilar coil is used.

Description and applications

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  1. parallel-wound, series connected
  2. parallel-wound, parallel connected
  3. counter-wound (series)
  4. counter-wound (parallel)

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The parallel-wound, series connected bifilar coil is how Nikola Tesla patented (512340) it. This way the capacity between the parallel windings is charged by the increased voltage difference (1/2 of the supply voltage) between the series connected windings. This makes it possible for the coil to hold a greatly increased amount of energy in its electric field, and lowers the resonant frequency of the coil drastically.

Some bifilars have adjacent coils in which the convolutions are arranged so that the potential difference is magnified (i.e., the current flows in same parallel direction). Others are wound so that the current flows in opposite directions. The magnetic field created by one winding is therefore equal and opposite to that created by the other, resulting in a net magnetic field of zero (i.e., neutralizing any negative effects in the coil). In electrical terms, this means that the self-inductance of the coil is zero.

The bifilar coil (more often called the bifilar winding) is used in modern electrical engineering as a means of constructing wire-wound resistors with negligible parasitic self-inductance. Large examples were used in inventor Daniel McFarland Cook's 1871 "Electro-Magnetic Battery" and Nikola Tesla's high frequency power experiments at the end of the 1800s. Nikola Tesla patented the Bifilar Coil on January 9, 1894, referring to it as a “Coil for Electro Magnets”.

</references>

  • Tesla's patent
  • Bifilar relay coils
  • Resistance coils for alternating current work By Harvey Lincoln Curtis, Frederick Warren Grover, United States. Bureau of standards. Reprint no. 177.

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