Such interference--which results from the transfer of electric energy from one telephone line to another, or from a power
line to a telephone line--is called inductive interference. Inductive interference may be either crosstalk or noise.
a. Crosstalk. Crosstalk is interference which results when two or more telephone talking circuits exist side by
side, and the conversation on one circuit may be heard on the others. Obviously, this is objectionable. It not only may
reduce intelligibility, but it may destroy secrecy. Crosstalk can result from any or all of the following causes:
(1) Conduction through leakage paths. Current in one line may be transferred to another line if the insulation
between lines is faulty, or if branches of trees or brush come in contact with the wires. This is a problem of proper
maintenance, and crosstalk from this cause can be eliminated by keeping the lines in good mechanical condition.
(2) Inductive coupling. One of the basic principles of electricity is that a magnetic field exists around a wire
through which current is flowing. The magnetic field consists of concentric circular lines of force at right angles to the
wire in space. The strength of the magnetic field varies inversely with the distance from the wire; that is, the greater the
distance, the weaker the magnetic field. The magnetic field has the same waveform as the current that produces it. If the
current is constant in magnitude and direction, the magnetic field is constant in strength and direction. If the current is
alternating, as it is in the case of voice frequencies, the magnetic field varies instantaneously in magnitude, and changes
direction every half-cycle. If such a varying magnetic field cuts an adjacent conductor, it induces an alternating emf in the
conductor, in accordance with the generator principle. The magnitude of this emf varies inversely with the distance
between the center of the magnetic field and the conductor. The conductor is said to be inductively coupled to the original
wire which produces the magnetic field. A, figure 62, illustrates how inductive coupling between adjacent telephone
circuits can cause crosstalk. Assume that an alternating voice-frequency current is flowing in circuit 1-2, which consists
of a telephone line connecting two telephone sets, T1 and T2 (represented by generators). At some instant of time, the
current is flowing in the direction indicated by the arrows. A magnetic field exists around both wires, causing the
production of a resultant magnetic field. When another telephone circuit, consisting of a line connecting
133
Previous Page
