74CHAPTER 3. REACTANCE AND IMPEDANCE – INDUCTIVEfrequency, Q does not vary proportionally with reactance. In order for a Q value to have precisemeaning, it must be speciﬁed at a particular test frequency.Stray resistance isn’t the only inductor quirk we need to be aware of. Due to the fact that themultiple turns of wire comprising inductors are separated from each other by an insulating gap (air,varnish, or some other kind of electrical insulation), we have the potential for capacitance to developbetween turns. AC capacitance will be explored in the next chapter, but it suﬃces to say at thispoint that it behaves very diﬀerently from AC inductance, and therefore further “taints” the reactivepurity of real inductors.3.6More on the “skin eﬀect”As previously mentioned, the skin eﬀect is where alternating current tends to avoid travel through thecenter of a solid conductor, limiting itself to conduction near the surface. This eﬀectively limits thecross-sectional conductor area available to carry alternating electron ﬂow, increasing the resistanceof that conductor above what it would normally be for direct current: (Figure 83,3.17)Cross-sectional area of a roundconductor available for conductingDC current"DC resistance"Cross-sectional area of the sameconductor available for conductinglow-frequency AC"AC resistance"Cross-sectional area of the sameconductor available for conductinghigh-frequency AC"AC resistance"Figure 3.17: Skin eﬀect: skin depth decreases with increasing frequency.The electrical resistance of the conductor with all its cross-sectional area in use is known as the“DC resistance,” the “AC resistance” of the same conductor referring to a higher ﬁgure resultingfrom the skin eﬀect. As you can see, at high frequencies the AC current avoids travel through mostof the conductor’s cross-sectional area. For the purpose of conducting current, the wire might aswell be hollow!