Very good write-up but...
In the telegraph line situation.... DC was used, with very few exceptions.
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DC cannot transmit information. And when DC is switched on and off, it is not DC but a rectangular signal.
As it is better known in radio circles "CW" (Continuous Wave) is modulation of an RF carrier only in that it turns the carrier off and on. Any sidebands from the transmitter are due to the unavoidable imperfections in the transmitting system.
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This is false. Having a long experience in radio transmissions both theoretical and practical, and in development of softwares for decoding radio data signals, including Morse code, I will try to explain.
"CW" is a way of speaking. A real continuous wave is not modulated, it is not switched on and off. Morse is switched CW.
To send Morse code, you need to switch on and off a signal, i.e. to generate a rectangular signal. Morse code specifications for best decoding are based on the dot duration (T). They recommend to use a dash of duration 3*T, a space of T between dash and dot inside a sign, a space of 3*T between signs, and 9*T between words.
T is around 50ms for operators transmitting at low/medium speed. If you transmit an H (....), this means that you send a squared signal of 100ms period = 0.1s (1 dot + 1 space, repeated 4 times). Therefore the associated frequency is 1/0.1 = 10Hz.
But it is not a sine signal. It is a squared signal, it has harmonics. To pass a rectangular signal, you need theoretically an infinite bandwidth due to the theoretically infinite time rise of the fronts (see "Fourier transforms"). In fact we can limit the bandwidth around 9 times the signal frequency without disturbing the signal shape too much. If an operator used a 10Hz passband filter, he will be unable to decode the signal due to the filter"ringing" that would make the instants of switching completely fuzzy. In practice, the "CW" filters in receivers have a bandwidth between 200-500 Hz. This allows the signal to keep its rectangular shape and the instants of switching to be precisely discriminated, what is absolutely needed for the readability.
Morse transmission is a 100% amplitude modulation of a carrier by a rectangular signal. It is a real amplitude modulation obeying the same rules as any amplitude modulation: we retrieve the spectrum of the input signal on each side of the carrier, in the upper and lower side band. It follows that we need twice the bandwidth of the input Morse signal, and adding the fact seen above that we need about 10 times the fundamental frequency of a squared signal to pass its harmonics for maintaining its shape, we understand the reason of a 200-500 Hz filter bandwidth for processing "CW" Morse signals in transceivers.
In conclusion any time you switch on/off a pure sine signal, you generate a wide spectrum. The sharper the switching, the wider the spreading. Any information transmission encoded by such a switching, occupies a non-zero bandwidth. In AM, the information is encoded in the two side bands around the carrier. "CW" Morse is AM by a rectangular signal, with 200Hz or more bandwidth.
A resonant circuit realizes a function of narrow filter whose the bandwidth is incompatible with a switching frequency that would be larger.
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