telephone sets T3 and T4, is located near the first circuit, as shown, the magnetic field of the first circuit links wires 3 and
4 of the second circuit (field not indicated). Because the magnetic field is varying instantaneously in intensity, an emf is
induced in each wire of circuit 3-4. The polarity of the induced emf is the same for both wires. Assume the polarity to be
as shown. Wire 3, however, is closer to the center of the magnetic field than wire 4, and, consequently, the emf induced
in wire 3 is greater than the emf induced in wire 4. An unbalanced emf (E3-E4) therefore exists in circuit 3-4, and
produces a corresponding current through sets T3 and T4. This current, having the same frequency variations as the
current in circuit 1-2, causes the conversation in circuit 1-2 to be heard in sets 3 and 4.
(3) Capacitive coupling. Capacitive coupling produces an unbalanced emf in a circuit because of the capacities
between the wires of an adjacent circuit and ground and the associated electric fields. This type of coupling between two
adjacent telephone lines is illustrated in B, figure 62, which shows a telephone circuit consisting of telephone sets T1 and
T2 connected by wires 1-2. The capacitance of these wires to ground are represented by CG1 and CG2, respectively.
capacitances are assumed to be equal, the voltages across them are equal in magnitude but opposite in polarity, as
indicated. This causes an electric field to appear between each wire and ground, with the respective directions indicated
by the arrows. If a second telephone circuit, consisting of sets T3 and T4 connected by wires 3-4, lies near the first
circuit, each of its wires is linked by the field existing at that point. Since the strength of the field diminishes with
increasing distance from the source, wire 3 is linked by a stronger electric field than that which links wire 4. This causes
wire 3 to be raised to a higher potential above ground than wire 4, so that a difference of potential exists between wires 3
and 4. This potential difference, or unbalanced emf, causes a current having the same frequency variations as the current
in circuit 1-2 to flow in circuit 3-4, and produces crosstalk in circuit 3-4. The closer the two circuits are to each other, the
greater is their susceptibility to this type of interference.
Interference in telephone lines caused by such sources as adjacent power
lines, electric motors and generators, and railway communication facilities, is classified as noise.
Electric noise results from the fact that the voltages in electric power lines and electric machines
and such sources are not pure 60-cycle sine waves, but also contain many other frequencies.