Unintentional radiator

Radio frequency signals used within the computer circuitry may be unintentionally coupled to the power cord or to an interconnecting cable, which then acts as an antenna.

Microprocessor-controlled appliances, anything with a clock signal, and switching voltage regulators all make some kind of noise, at the repetition frequency and at harmonics.

This prevents leakage from cable television systems, for example, from interfering with radio communications between aircraft and control towers.

It also generally means that users who intentionally radiate signals (TV stations and cell phone companies) can order the device turned off if it interferes with their licensed operations.

The big cylindrical bumps on the cable to monitors and laptop chargers are ferrite cores which reduce undesired signals.