Page 438 from "The Making of the Atomic Bomb", by Richard Rhodes.
"Fermi instructed Weil to move the cadmium rod to a position which was about
half-way out. [The adjustment brought the pile to] well below critical
condition. The intensity rose, the scalers increased their rates of clicking
for a short while, and then the rate became steady, as it was supposed to.
Fermi busied himself at his slide rule, calculating the rate of increase,
and noted the numbers on the back. He called to Weil to move the rod out
another six inches. Again the neutron intensity increased and leveled off.
The pile was still subcritical. Fermi had again been busy with his little
slide rule and seemed very pleased with the results of his calculations.
Every time the intensity leveled off, it was at the values he had
anticipated for the position of the control rod."
Date was December 2, 1942. If you have not read Rhodes book, I highly
recommend it. A well written history of the people and the world at that
time in history. Best regards, Bill