continuous wave ("cw")
Following the discovery of electromagnetism in 1820 and the invention of the electromagnet in 1824, there were developments in electromagnetic telegraphy in Europe and America. Pulses of electric current were sent along wires to control an electromagnet in the receiving instrument.
In 1895, Guglielmo Marconi developed the first radio communication system, using a spark-gap transmitter to send code over long distances. (In December 1901, he first transmitted a signal across the Atlantic Ocean from Cornwall, England, to Newfoundland, Canada). The very first radio transmitters used this spark gap technology to produce radio-frequency oscillations in the transmitting antenna. Spark gap transmitters produced brief transient pulses of radio waves called damped waves spread over an extremely wide band of frequencies (wide bandwidth). As a result, they produced electromagnetic interference (RFI) that spread over transmissions of stations at other frequencies. Nevertheless, this technology was used in the first three decades of radio.
Then it was realized that the ideal radio wave for this purpose would be a sine wave with zero damping (a continuous wave or ”CW”). An unbroken continuous sine wave theoretically has no bandwidth so all its energy is concentrated at a single frequency. Continuous waves cannot be produced with an electric spark, but were initially produced with a vacuum tube electronic oscillator.
In order to transmit information, the continuous wave must be turned off and on with a telegraph key to produce the different length pulses, "dots" and "dashes", that spell out text messages in Morse Code.
Initially, all amateur radio communication was by Morse Code since phone (voice) communication had not yet been developed. The code currently used by amateur operators is the International Morse Code (there were several predecessor forms of code). Early hams, (and even many of today’s hams) had to learn Morse Code to obtain their license. Five words per minute was the requirement for the entry level Novice license. A General or Advanced class required 13 wpm and it was 20 wpm for an Extra.
In 1991 the code requirement was eliminated for the Technician class, in 2000 it was reduced to 5 wpm for all classes that still required code and in 2007 the code requirement was eliminated entirely.
But while the licensing requirement went away, learning and using Morse Code and CW technology has its advantages and is a fun mode for many operators who practice it.
By concentrating all the transmitted energy within a vary narrow bandwidth, CW gains a huge advantage in the areas of signal to noise ratio and power density. This means it gets through better, given the same antenna and transmitter power. A CW signal occupies roughly 100Hz of spectrum, whereas SSB uses around 2000Hz. At those bandwidths, a 5W CW signal has the same power density as a 100W SSB signal. And CW band privileges extend across almost all the amateur radio spectrum with very few exceptions.
There are a number of other advantages and considerations. Please visit AARL's Learning Morse Code page for more information and resources on this topic. Also popular is the Koch Method of learning Morse Code. This revolutionary teaching approach was invented in 1935 by the German engineer Ludwig Koch. It uses the full target speed from the outset but begins with just two characters. Once strings containing those two characters can be copied with 90% accuracy, an additional character is added, and so on until the full character set is mastered.