668 Chapter 22
Shielded, twisted pairs in balanced circuits are an absolute necessity in handling low-
level signals in order to avoid electromagnetic interference. The braided shield alone
offers protection from electrostatic fi elds but offers very little protection from changing
magnetic fi elds. The practice of employing twisted pair conductors stems from experience
gleaned from the early days of the telephone industry.
In former times, long distance circuits between cities and local circuits in rural areas
employed open bare wire pairs affi xed to separate glass insulators attached to multiple
cross arms, which were in turn elevated by poles. It was learned early on that open-air
electrical power lines that often followed parallel paths caused interference. It was found
that by periodically transposing the positions occupied by the two conductors of a given
circuit pair, that the interference could be reduced greatly if not eliminated altogether.
This transposition amounted to periodically twisting without touching one conductor of
a circuit pair over the other, in effect forming an insulated, twisted pair even though the
distance between twists was relatively large. The explanation for this annulment of the
interference appears in Figure 22.15.
In Figure 22.15 imagine that the twisted pair of conductors is replicated to the right as
well as to the left to form an extended circuit. Imagine also that in the vicinity a magnetic
fi eld is instantaneously directed into the fi gure as indicated by the X’s and that the fi eld
strength is increasing with time. Examine the two closed paths as indicated by the circles.
According to Lenz’s law, the induced voltage acting in the loops has the sense indicated
by the arrows. Now look at the white conductor in the upper left, the induced voltage in
this portion of conductor is in the same direction as is the arrow adjacent to it. Compare
Twisted pairX XFigure 22.15 : Twisted pair exposed to a time-changing magnetic fi eld.