All copper wires have internal resistance to signal flow, called impedance. Furthermore, telephone wires develop capacitance between the wires, which means the wires store an electrical voltage, similar to a battery. When the stored voltage builds up to an excessive level, it discharges rapidly, causing a voltage spike. This stored voltage interferes with the electrical signals used to carry voice communication. For a short run, the interference effects of capacitance and impedance are minimal. For longer runs, the interference becomes significant, resulting in the eventual total loss of signal. To compensate for the interference effects of impedance and capacitance, engineers have designed the H88 load coil. This is a simple coil of wire, built into the telephone lines, which creates a tuned resonance circuit to cancel out the negative effects of capacitance and impedance. When the internal capacitance of the wire discharges an unwanted voltage spike, the H88 load coil absorbs the discharge. Since the load coil is tuned to the same frequency as the capacitive voltage spike discharge, it's said to be in resonance with the circuit, tuning out the negative effects of capacitance and impedance.
Engineers have determined exactly where to put H88 load coils in a phone line. After much research, they've found it best to place coils every 6,000 feet in a phone line. Loss measurements are taken between each load coil, and the losses cannot be more than specified for each frequency range. For example, the loss cannot be more than 10 percent at 1,000 Hz. Human hearing is about 20 to 20,000 Hz, so this is about the range of a human voice.
Because load coils were designed for telephone frequencies, they're incompatible with digital signals. Load coils block high frequencies. Unfortunately, digital computer signals use the frequencies load coils were designed to block. Because of this, the industry is phasing out H88 load coils and copper telephone wires as of 2010. Coaxial cable and fiber optics, which have none of the impedance problems associated with wire telephone lines, are replacing the phone lines. As of late 2010, the H88 system of load coils on telephone lines is antiquated technology.