Cryptonomicon

by Neal Stephenson

Long before this, William Thomson had figured out, by dint of Fourier analysis, that incoming bits could be detected much faster by a more sensitive instrument. The problem was that instruments in those days had to work by physically moving things around, for example, by closing an electromagnetic relay that would sound a buzzer. Moving things around requires power, and the bits on a working transatlantic cable embodied very little power. It was difficult to make a physical object small enough to be susceptible to such ghostly traces of current. Thomson’s solution (actually, the first of several solutions) was the mirror galvanometer, which incorporated a tiny fleck of reflective material that would twist back and forth in the magnetic field created by the current in the wire. A beam of light reflecting from the fleck would swing back and forth like a searchlight, making a dim spot on a strip of white paper. An observer with good eyesight sitting in a darkened room could tell which way the current was flowing by watching which way the spot moved. Current flowing in one direction signified a Morse code dot, in the other a dash. In fact, the information that had been transmitted down the cable in the brief few weeks before Wildman Whitehouse burned it to a crisp had been detected using Thomson’s mirror galvanometer — though Whitehouse denied it.