Interference and Noise in Wire Communication Systems

Sources of interference may be classified as natural or artificial. The natural classification includes lightning, static, the aurora borealis phenomena, and dust storms phenomena. Artificial sources include power lines, electric railway systems, and other communication circuits. Induced currents from these and other sources cause noise in the telephone receivers. An unusual source of noise is explained in reference 6.

Telephone lines are designed for, and operated with, low voltages and currents and must therefore be protected not only from the induced noise currents just considered but also from high-voltage natural and artificial hazards as well. Lightning offers the greatest natural hazard and, through either direct or induced strokes, can injure or destroy telephone equipment. Heavy earth currents that accompany magnetic storms and auroral displays have damaged grounded apparatus and cables. Power lines, including electric trolleys and feeders, offer the greatest artificial hazards. These lines may come in direct contact with telephone circuits, or may induce relatively high voltages into these circuits as this chapter will explain. Electrolysis also is a source of considerable trouble.