Radio-frequency equipment is used for a vast range of purposes, including heat treating special steels, medical diathermy treatment for cancer, heat sealing plastic bags, and experiments in atomic physics. Nevertheless, as the name implies, the original use was in connection with the transmission of information by radio waves. The earliest form of this was wireless telegraphy (WT) using Morse code. This was followed by wireless telephony and, much later, broadcasting—radio and television. So, before diving into RF circuits in detail, a word might be in order about the different forms of modulation employed to impress the information to be transmitted onto the radio wave.