coil is used as a means of removing the energy stored in the stray magnetic flux which fails to link the primary coil to the secondary coil of the transformer. Because of their proximity, the wires of the bifilar coil both "see" the same stray magnetic flux. One wire is clamped to ground usually by a diode so that when the other "primary" wire of the bifilar coil no longer has a voltage applied across it by the switching transistor, the stray magnetic flux generates a current in the clamping coil with the primary side voltage appearing across it, causing an equal voltage to appear across the primary winding. If this clamping coil were not used, the stray magnetic flux would attempt to force a current to flow through the primary wire. Since the primary wire is switched off and the switching transistor is in a high resistance state, the high voltage which would appear on the semiconductor switching transistor would exceed its electrical breakdown or even damage it. Bifilar coils impose an inductance in the common mode, but impose no inductance in the differential mode. Coils in such a combination are widely used to eliminate ingress or egress of common mode signals from electronic signalling circuits. This arrangement is used in transmission and reception magnetics of Ethernet cables and conspicuously in the form of a ferrite bead clamped to the outside of USB, laptop power supply and HDMI cables. History German physicist Wilhelm Eduard Weber made use of the bifilar coil in his 1848 electrodynamometer. 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”. See also Ayrton-Perry winding Basket winding Mirror bifilar electrodynamometer References External links 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. Electromagnetic coils