This is from a little booklet about the first atomic pile...(sorry
about the length)

From "The First Reactor" U.S. Atomic Energy Commission, 1968

At 2:50 the control rod came out another foot. The counters nearly
jammed, the pen headed off the graph paper. But this was not it.
Counting ratios and the graph scale had to be changed.
"Move it six inches," said Fermi at 3:20. Again the change - but again
the leveling off. Five minutes later, Fermi called: "Pull it out
another foot."
Weil withdrew the rod.
"This is going to do it, Fermi said to Compton, standing at his side,
"Now it will become self-sustaining. The trace will climb and
continue to climb. It will not level off."
Fermi computed the rate of rise of the neutron counts over a minute
period. He silently, grim-faced, ran through some calculations on his
slide rule.

In about a minute he again computed the rate of rise. If the rate was
constant and remained so, he would know the reaction was
self-sustaining. His fingers operated the slide with lightning speed.
Characteristically, he turned the rule over and jotted down some
figures on its ivory back.

Three minutes later he again computed the rate of rise neutron count.
The group on the balcony had by now crowded in to get an eye on the
instruments, those behind craning their necks to be sure they would
know the very instant history was made. In the background could be
heard Wilcox Overbeck calling out the neutron count over an
annunciator systern. Leona Marshall (the only girl present),
Anderson, and William Sturm were recording the readings from the
instruments. By this time the click of the counters was too fast for
the human ear. The clickety-click was now a steady brrrrr. Fermi,
unmoved, unruffled, continued his computations.


The Curve is Exponential

"I couldn't see the instruments," said Weil. "I had to watch Fermi
every second, waiting for orders. His face was motionless. His eyes
darted from one dial to another. His expression was so calm it was
hard. But suddenly, his whole face broke into a broad smile."
Fermi closed his slide rule -
"The reaction is self-sustaining," he announced quietly, happily.
"The curve is exponential."
The group tensely watched for twenty-eight minutes while the world's
first nuclear chain reactor operated.
The upward movement of the pen was leaving a straight line. There was
no change to indicate a leveling off. This was it.
"O.K., 'Zip, in," called Fermi to Zinn who controlled that rod. The
time was 3:53 p.m. Abruptly, the counters slowed down, the pen slid
down across the paper. It was all over.
Man had initiated a self-sustaining nuclear reaction - and then
stopped it. He had released the energy of the atom's nucleus and
controlled that energy.


In the same booklet is a photograph of Laura and Enrico Fermi sitting
at a table, both looking at the slide rule in Enrico's hands. It is a
duplex, probably K&E, with apparently the dark wood (mahogany?).

Fun stuff!

Tom Pugsley