A bi-filar coil has an output which contains two different phases. The phase angle depends on the relative lengths of the two windings. With both wires the same length, the phases are at 180. The output wire could keep on winding, by itself, until that wire is, for instance, a quarter wavelength longer than the input wire which is wrapped side by side throughout its shorter length. Nikola's patent shows the outer wire extending an extra 90 degrees around the perimeter, but this doesn't necessarily give rise to a 90 degree phase difference in the signal. That wire could wrap on around a couple of times, or more, depending on the desired phase variance, and the overall wire lengths.
When both wires have the same length, running the single wire output through a full wave rectifier will double the intensity of the potential. Or a single LED on the output will chop the bottom of the phases, leaving the same biphasic frequency. (With a DC offset, both complete phases will pass through the cell).
The effect the phase variance has depends on the substance. A given substance will require a characteristic phase angle. If all you want is a longitudinal wave, use 90 degrees. Then each molecule will experience a field which compresses, then expands, as each wave passes.
