Development of Loading

Pupin successfully solved the problem of what is called inductive loading. He suggested adding series inductance, in the form of carefully constructed coils, at regular and comparatively short intervals.

At about this same time,¹⁴ Campbell, working independently, also developed a theory of inductive loading. Priority was adjudged Pupin, and his patent rights were acquired by the Bell System,¹⁵ whose engineers are largely responsible for the application of loading to modern communication.

Loading was first applied to telephone open-wire lines in about 1900. By the use of loading, talking distances were approximately doubled. By 1925, most loading in the United States had been removed from open-wire lines. This was because of the development of the vacuum-tube amplifier. Loading is expensive, and on open-wire lines is particularly susceptible to impairment and damage by heavy transient currents, such as are induced sometimes by lightning. At the present time, only cables are loaded in the United States.