The signals were sent by cutting up the continuous hum in the telephone into long and short periods in accordance with the Morse code by manipulating the key in the primary circuit.
In this manner it was possible to hear a Morse code dash or dot in the telephone.
Hence according as the trains of oscillations are long or short so is the sound heard in the telephone, and these sounds can be arranged on the Morse code into alphabetic audible signals.
At the moment I am building a Morse code reader to interface to my 64K ZX-81 using a tone decoder and A/D converter.
Others were in Morse code or started with electronic music, similar to a Stylophone or musical doorbell.
