"
"That I saw," Hamilton answered, "and I could see, of course, that
this was one of the most important parts of the machine, and that upon
it a good deal of the exactness of the work depended."
[Illustration: TABULATING MACHINE. Mechanism whereby the punched cards
are verified and every error prevented, and which also tabulates and
numbers all records taken. (_Courtesy of the Bureau of the Census._)]
"It does," the other replied. "Now if you look into those holes in the
plate you can see a little cup of bright metal under each hole. What do
you suppose that is?"
"I'm not sure, of course," the boy responded, "but it looks very much
like quicksilver."
"That's exactly what it is, quicksilver, or mercury. Now mercury, you
ought to know, can transmit an electric current, so that if an
electrically charged pin comes down into the cup of mercury, the cup
itself being attached to an electric current, a circuit is formed."
"Now I'm beginning to see," the boy said, "but what is the idea of the
cup of mercury; could not the pin just as well touch on a metal plate?"
"It could, of course, but a piece of dust between would prevent contact,
the pins would wear away quickly, and the plate would get worn, whereas,
by the pin just dropping into the mercury there is no friction and no
fear of a missed contact.
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