What Is a Bifilar Pancake Coil? Tesla's 1894 Design Explained
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A bifilar pancake coil is a flat spiral wound from two parallel insulated conductors connected in series.
"Bifilar" means two wires wound side by side. "Pancake" means the coil is formed as a flat spiral in a single plane rather than as a cylinder or helix.
The point of the arrangement is the voltage between neighboring turns. Because the coil is wound from two conductors joined to each other rather than run as one continuous wire, any two adjacent turns sit at very different points in the circuit. In an ordinary coil, neighboring turns differ by a fraction of a volt. In this winding, they can differ by half the full coil voltage. Since every pair of adjacent turns acts as a small capacitor, that large voltage difference gives the coil a high self-capacitance built into its own structure.
Nikola Tesla patented this construction in U.S. Patent 512,340, "Coil for Electro-Magnets," issued January 9, 1894, on an application filed July 7, 1893. His purpose was to build the needed capacitance directly into the coil so that separate capacitors would not be required.
What Tesla Was Trying to Solve
In alternating-current circuits, the self-induction of ordinary coils creates unwanted effects. The usual fix in the 1890s was to add separate condensers to the circuit. Tesla wanted to eliminate them.
From the patent:
"My present invention has for its object to avoid the employment of condensers which are expensive, cumbersome and difficult to maintain in perfect condition, and to so construct the coils themselves as to accomplish the same ultimate object."
His method was to change the winding itself:
"In order to attain my object and to properly increase the capacity of any given coil, I wind it in such way as to secure a greater difference of potential between its adjacent turns or convolutions."
Tesla is explicit about the mechanism. The coil keeps its self-induction. What the winding adds is enough capacitance to balance it:
"I have found that in every coil there exists a certain relation between its self-induction and capacity that permits a current of given frequency and potential to pass through it with no other opposition than that of ohmic resistance, or, in other words, as though it possessed no self-induction."
That is electrical resonance between the coil's own inductance and its own capacitance, at one particular frequency. The coil behaves as though it has no self-induction at that frequency. It does not physically lose its inductance.
The Number That Explains the Design
Tesla gave a clear numerical example in the patent.
Take a coil of 1,000 turns with 100 volts across its terminals. Wound conventionally, the voltage difference between two adjacent turns is about 0.1 volt. Wound bifilar — a second conductor laid parallel to the first, with the end of the first joined to the start of the second — the difference between adjacent points becomes 50 volts.
Energy stored between the turns is proportional to the square of that voltage difference. Fifty volts instead of a tenth of a volt is a 500-fold increase, and 500 squared is 250,000. Tesla wrote:
"the energy stored in the coil as a whole will now be two hundred and fifty thousand as great."
He also noted a practical advantage:
"Capacity secured in this particular way possesses an additional advantage in that it is evenly distributed."
Two Bifilar Windings, Two Different Coils
Bifilar windings come in two forms that behave in opposite ways, and the difference is almost never explained clearly. Both are called "Tesla bifilar pancake coils" online. Only one of them is what the patent describes.
Series-aiding — what the patent describes. Tesla writes that "the end of A be connected with the starting point of B." Both conductors are then traversed in the same rotational direction. The magnetic fields add and the coil keeps its inductance. Only the voltage distribution changes, which is the whole point: the built-in capacitance is there to resonate with an inductance that still exists. Tesla's claim is a coil with "a capacity capable of neutralizing its self-induction." If the winding cancelled the magnetic field, there would be nothing left to neutralize.
Center-joined — what is usually built today. Two interleaved spirals are joined at the center, forming one unbroken conductor that enters at one edge and leaves at the other. Current travels inward through the first spiral and outward through the second, so it circulates in opposite directions. The magnetic fields largely oppose each other and net inductance drops sharply. Cancellation is not total, because at any given angle the two spirals sit at slightly different radii.
Both windings produce the large turn-to-turn voltage difference and the high self-capacitance Tesla was after. They differ in what happens to the magnetic field.
There is a third arrangement worth naming to avoid confusion: the counter-wound non-inductive bifilar, where a wire is folded in half and both halves wound together. That is how non-inductive wirewound resistors are made. It cancels the field but does not produce the large turn-to-turn voltage difference, because the folded halves sit at nearly the same potential where they are closest.
Bifilar Winding vs. Ordinary Winding
| Feature | Ordinary coil | Series-aiding bifilar (patent) | Center-joined bifilar |
|---|---|---|---|
| Conductors | One | Two, joined end-to-start | Two interleaved spirals, joined at center |
| Current direction | Same throughout | Same rotational direction in both | Opposite rotational direction |
| Magnetic field | Reinforces itself | Reinforces itself | Largely opposes and cancels |
| Self-inductance | Normal | Normal | Strongly reduced |
| Voltage between adjacent turns | Very small | Very large | Very large |
| Self-capacitance | Low | High, evenly distributed | High, evenly distributed |
| Separate capacitor needed | Yes | No | No |
Why "Pancake"
A helical or solenoid coil is wound as a cylinder. A pancake coil is wound as a flat spiral in one plane. The geometry places all turns in a single plane and lets the coil sit flat against a surface.
A bifilar pancake coil combines both features: two parallel conductors wound as a flat spiral and connected in series.
Bifilar pancake coils are still used today in high-frequency and high-voltage experimental work, including as accessories for certain plasma devices.
How Bifilar Pancake Coils Are Used With Plasma Machines
Plasma devices such as the TZLA plasma machine output high-frequency, high-voltage energy through accessories connected to two poles.
A bifilar pancake coil connects to one pole as an accessory. Its flat form allows a large contact area against a surface, and its electrical behavior differs from a cylindrical coil or a gas-filled bulb: the center-joined winding gives it high distributed capacitance and strongly reduced net inductance, so it presents a largely capacitive load rather than an inductive one.
These are experimental and educational accessories. They are not medical devices and make no therapeutic claims. Faro Wolf is an independent maker and is not affiliated with, endorsed by, or partnered with TZLA or TZLA.club.
Building One — Practical Specifications
The figures below are from Faro Wolf's own production of the Gandr bifilar pancake coil in Norway.
| Parameter | Value |
|---|---|
| Overall diameter | 265 mm (10.4 in) |
| Winding diameter | 250 mm (9.8 in) |
| Thickness | 5 mm (0.2 in) |
| Conductor length in winding | 10 m (32.8 ft) |
| Conductor | Fine-stranded OFC copper |
| Body material | TPU 85A, flexible |
| Winding pattern | Two interleaved spirals, joined at center, one continuous conductor |
The two spirals are drawn in a single plane and rotated 180° relative to each other, so they interleave in the same radial space. They are joined at the center, which means the finished coil is a single unbroken conductor entering at one edge and leaving at the other. This is the center-joined form described above: high distributed capacitance, strongly reduced net inductance.
Groove width is the constraint that determines turn count. The measured outer diameter of the conductor sets the groove, and the groove pitch sets how many turns fit in the available radial space.
Frequently Asked Questions
What does bifilar mean? Bifilar means "two threads." A bifilar coil is wound from two parallel conductors rather than one. How they are connected determines how the coil behaves: joined end-to-start, the magnetic fields add; joined at the center, current circulates in opposite directions and the fields largely cancel.
What is the difference between a bifilar coil and a pancake coil? They describe different things. Bifilar describes the number of conductors; pancake describes the shape. A bifilar pancake coil is both — two conductors wound as a flat spiral.
Did Tesla invent the bifilar coil? Tesla patented a specific application of it. He notes in the patent that coils of independent strands wound side by side "are not in themselves new," but states that the purpose and result he describes had not previously been recognized. The patent is U.S. 512,340, granted January 9, 1894.
Why does a bifilar coil have low inductance? Only the center-joined form does. In that winding the two spirals carry current in opposite rotational directions, so their magnetic fields oppose and largely cancel, leaving behavior dominated by the capacitance between the closely spaced conductors. The series-aiding winding described in Tesla's patent keeps its inductance — the capacitance is there to resonate with it, not to remove it.
What is a bifilar pancake coil used for? Historically, to build capacitance into a coil and avoid separate capacitors. Today it appears mainly in high-frequency and high-voltage experimental work, including as an accessory for plasma devices.
How large is a typical bifilar pancake coil? There is no standard size. The Gandr made by Faro Wolf measures 265 mm overall with a 250 mm winding and is 5 mm thick.
Are bifilar pancake coils medical devices? No. They are electrical accessories sold for experimental and educational use. They are not intended to diagnose, treat, cure or prevent any disease.
Sources
Nikola Tesla, U.S. Patent 512,340, "Coil for Electro-Magnets," filed July 7, 1893, granted January 9, 1894. Full text: https://patents.google.com/patent/US512340A/en