Tesla Bifilar Coil DC Resistance: Pancake (1)

Here I test the DC resistance of a Tesla Bifilar Pancake coil (Tesla’s patent #512340) using the Fluke 87-III digital multimeter in its High Precision mode.

First I test the entire coil, then I test each of the two windings separately. The DC resistances of the two windings are each one-half of the total resistance of the entire coil, as expected.

Claims have been made that there is something special happening to the DC resistance of Tesla Bifilar-wound coils, whether flat or solenoidal. Specifically, that the TBF winding has half the DC resistance of a monofilar coil with the same total length of wire. At the moment I don’t have a matching monofilar coil to compare, but I can do the next best thing: measuring each winding separately in this TBF coil and comparing that to the measurement of the whole coil.

I have also measured solenoidal coils where I do have TBF and monofilar windings of the same total length, and they have the same DC resistance, so at least in the coils I have tested thus far, the “half resistance” claim is not supported by my findings.

With the same wire being used, it should not matter at all whether the wire is straight, wound in a monofilar coil or a bifilar coil or even bunched up like a brillo pad; the total DC resistance should only depend on the total length of the wire.

However, it is trivially easy to mis-connect the crossover center tap when making the Tesla Bifilar winding, simply by getting the wires mixed up, so that the top of one winding is connected to the bottom of the _same_ winding. This isolates this half of the coil from the measurement and gives the false reading of half the expected total resistance in the other half of the coil. Could a mistake like this be where the “half resistance” claim comes from?

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