City of Light: The Story of Fiber Optics

82 CITY OF LIGHT
Figure 7-2: Alexander Graham Bell’s photophone modulated reflected sunlight
with sound vibrations, then detected the sound by using the light to illuminate a
piece of selenium. It carried voices without wires, but not very far. (Courtesy
Lucent Technologies)
graphing ‘‘SOS’’ for help. Progress came with the advent of electronic circuits
tuned to oscillate at specific frequencies so that several transmitters could
share the spectrum.
Radio began at low frequencies, then moved relentlessly higher. In the
early 1920s, Hansell helped build the first radio transmitter to send voices
reliably across the Atlantic. 14 It oscillated 57,000 times a second—57 kilohertz
in radio parlance—generating what were called ‘‘long waves’’ because
each one stretched 5.26 kilometers (3.27 miles). At night, both the ionosphere
and the ocean reflect long waves, so they could carry whole speeches
across the Atlantic, something not possible with earlier transmitters. 15
Vacuum tubes soon made it possible to generate higher frequencies reflected
better by the ionosphere. Soon after Hansell established the RCA Rocky