In front of this beam
whirled a five-spindled wheel governed by a chronometer which was
so accurate, he said, that it erred only a second a day.
Between the poles of the galvanometer was stretched a slender
thread of fused quartz plated with silver. It was the finest
thread I could imagine, only a thousandth of a millimeter in
diameter, far too tenuous to be seen. Three feet further away was
a camera with a moving plate holder which carried a sensitized
photographic plate. Its movement was regulated by a big fly-wheel
at the extreme right.
"You see," remarked Garrick, now engrossed on the apparatus and
forgetting the hammer evidence for the time, "the beam of light
focussed on that fine thread in the galvanometer passes to this
photographic plate. It is intercepted by the five spindles of the
wheel, which turns once a second, thus marking the picture off in
exact fifths of a second. The vibrations of the thread are
enormously magnified on the plate by a lens and produce a series
of wavy or zigzag lines.
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