This was long before there were any electronic devices to amplify or otherwise manipulate the signal. The telegraph signals were just short electrical pulses r

This was long before there were any electronic devices to amplify or otherwise manipulate the signal. The telegraph signals were just short electrical pulses representing Morse code, with a positive voltage for a dot and a negative voltage for a dash. The lines could only be worked in one direction at a time for many years, but eventually circuits were used at each end to "duplex" the cable, allowing messages to be sent in both directions at the same tim.