 excellent insulator. However, we well know that air can pass electricity if the voltage is very high. Consider lightning, for example. Other examples of insulators are bakelite, celluloid, fiber, formica, glass, lucite, mica, paper, rubber, and wood. Figure 2.10 A simple capacitive circuit. 4A capacitor is a device whose primary purpose is to introduce capacitance into a circuit. SIGNALS CONVEY INTELLIGENCE The insulated conductors of every circuit, such as our wire pair, have to a greater or lesser degree this property of capacitance. The capacitance of two parallel open wires or a pair of cable conductors of any considerable length is appreciable in practice. 2.5.1.2 Bandwidth (Hz) of a Twisted Pair. The usable bandwidth of twisted wire pair varies with the type of wire pair used and its length. Ordinary wire pair used in the PSTN subscriber access plant can support 2 MHz over about 1 mile of length. Special Category 5 twisted pair displays a 67-dB loss at 100 MHz over a length of 1000 ft. 2.5.1.3 Bandwidth Defined. The IEEE defines bandwidth as "the range of frequencies within which performance, with respect to some characteristic, falls within specific limits." One such limit is the amplitude of a signal within the band. Here it is commonly defined at the points where the response is 3 dB below the reference value. This 3-dB power bandwidth definition is illustrated graphically in Figure 2.11. 2.5.2 Coaxial Cable Transmission Up to this point we have been discussing two parallel conductors, namely, wire pair. An entirely different configuration of two conductors may be used to advantage where high and very high radio frequencies are involved. This is a coaxial configuration. Here the conducting pair consists of a cylindrical tube with a single wire conductor going down its center as shown in Figure 2.12. In practice the center conductor is held in place accurately by a surrounding insulating material that may take the form of a solid core, discs, or beads strung along the axis of the wire or a spirally wrapp