Amplitude modulation

Amplitude Modulation, usually shortened to AM, is a simple way to send a radio signal. The signal can travel long distances, and appear in faraway places, because of the earth's ionosphere. 
Amplitude is the height of a wave. A carrier wave is modulated (changed) by adding audio. The combined signal is transmitted over a frequency. AM signals can be received by a crystal-powered radio set, that needs no batteries or electrical power source. Many early radios were homemade, and used earphones.
AM signals are mostly sent out on medium wave and shortwave frequencies. They do not carry high fidelity sound, but the sound is good enough to hear most things. Analog signals can fade in and out, or have interference from power lines and power stations. Sunspot activity can also weaken signals.
In the early years of commercial broadcasting in the 1920s, medium wave AM (from 540 to 1600 kilocycles) was the only kind of radio widely used. Radio programs were much like today's television programs, including news, sports, drama, music, comedy, and other entertainment.
Later Frequency Modulation (FM) radio was developed. It had better fidelity, but the signals could not travel as far, so AM was still important. After television became common in the 1950s, AM became a major format for popular music. As FM became common in the 1970s, AM stations featured more news, and talk shows.
Shortwave radio signals (from 3000 to 30,000 kilocycles) can travel around the world, and are used by countries to send programs to distant places. These programs feature news about the country, examples of their native culture and history, and sometimes foreign language lessons.