Certain factors limiting the volume efficiency of repeatered telephone circuits

Vacuum tube amplifiers are now regularly built into long distance telephone circuits where required to maintain their volume efficiency. Consequently, the overall volume efficiency of these circuits no longer depends to any important extent on the loss per unit length of the line wires. Instead, the efficiency is controlled by certain factors which, before amplifiers were introduced, had negligible effect. Among these factors are echo, singing or “near singing,” and crosstalk. The stability of the lines and amplifiers also becomes very important.