the left, there is a 1866 book "Electric
Telegraph" by George B. Prescott published in Boston,
Massachusetts. It was a long way to the invention of
"Radio", so this book was devoted to all the
different wired telegraphy systems know by the middle of XIX
the right, there is a 1902 book "Telegraphy
Self-Taught" by Theo A. Edison published in Chicago,
Illinois. The book is open right on the picture of so-called
Combination Key consisting of a Straight Key and a Sounder
producing a 1000 Hz sounds when key on the other end of wire
line was depressed.
the second half of XIX century and early 1900s, the
telegraph operators have been rare experts with very decent
salaries. Each of them had his personal "Combination
Key" which they connected to telegraph wire lines.
Messages were transmitted and received just manually.
between the books there is a real sample of such "Combination Key"
shown. It was manufactured presumably by very early 1900s.