"They would have to live in the same district, they would have to be
employed the same way, they would have both to be married and have the
same number of children and a whole lot more things, and even then--the
cards would be different for they would represent different numbers on
the schedule on which their names were registered. No, there are not two
cards in the entire series punched alike."
"Then I don't see how in the wide world this machine can tell which
cards are right among millions so entirely different from each other."
"They don't verify by finding the cards that are right," was the answer,
"but by picking out the cards that are wrong."
"What's the difference?"
"There is a wide difference. You can see that it would be easy enough to
arrange that machine so that if a wrong combination of contacts were
made the bell would not ring. Such wiring might be highly complex, but
you see the idea is simple. For a right group of contacts, all the wires
are satisfied, as it were, and the bell rings; for an error, one wire,
cut in on by a wrong wire, breaks the contact, and the bell does not
ring."
"But what do you mean by a wrong grouping?" asked the boy.
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