For years after becoming a ham I asked why CW was called CW. The answer was always the same, “CW means continuous wave” but it is not continuous because by nature Morse code is made up of broken carrier or wave. When I continued with this line of thought again I would get a single answer, “I don’t know why.” Oh if I had just talked to a ham who had been one for long enough to have used a Spark Gap transmitter I would have received my answer.
The first wireless transmitters were basically nothing more then a voltage source, a coil, an arc or spark gap, a key, an antenna, and a ground. The coil was frequently a spark coil from a model T Ford. The transmitted signal was over the whole RF spectrum. Later tuned circuits were added which suppressed the RF signals generated above and below a given wave length but the output was still a very wide portion of the band.
Another problem with the Spark Gap transmitter was the signal would dampen out. The signal could not be sustained for a long period and thus the spark must be continuously interrupted and restarted. This meant a lot of noise but that was good because the receivers were only passive devices. They had no amplification just a detector and those early detectors were not all that efficient. Certainly they did not have a bfo but with the white sound produced along with the interrupt/restart frequency there was a sufficient noise in the signal to copy.
After Lee de Forest added a grid to the Fleming valve, now called vacuum tube diode, in 1906 he found it could be used to amplify a signal. This allowed the construction of the regenerative receiver and soon the audion, the name de Forest gave his triode, was also used to as an oscillator to generate the transmitted signal. The first ham stations using this new form of generating and detecting an RF signal usually consisted of a single audion being used as both the transmitter and the regenerative receiver.
The definition for Continuous Wave is a single frequency sustained at a constant level. If a carrier is modulated it will contain sidebands and in the case of FM not only sidebands but also the frequency of the carrier itself changes. Thus to qualify as CW it must be unmodulated. CW was also known as “undamped waves”, to distinguish damped wave generated by the Spark Gap transmission.
There were a few hams that experimented with trying to modulate spark gap transmitters but for the most part all wireless communications were by Mores Code.
Now the continuous wave could be sent but it contained no intelligence. To use it for communications it had to be interrupted into dots and dashes thus the true name for this communications was actually Interrupted Continuous Wave or ICW but the I was dropped and it became just CW, the necessity for the keying was understood.
There was a period of time where there were both CW and Spark Gap signals on the air. Those using Spark Gap usually had passive detectors and thus were unable to read the CW signals so amplitude modulation was added making it Modulated Interrupted Continuous Wave or IMCW which was shortened to MCW.
Now if someone asks you why radio Morse code is called CW you can tell them it is short for ICW.