The Special Fish Report
Albert W. Small (December 1944)
Page 60 1/2
Codes and Ciphers
TOP SECRET Special Fish Report Page 60 1/2
Psi one and psi two therefore set with a colossal bang.
When the limitation is a P5twoback limitation, a combined motor
and psi run is made. Naturally the best setting in the motor run will
result when psi-five is set correctly, and so psi-five and motor can
be set simultaneously. Then psi one and two follow as before.
The difficulty with P5twoback limitation is of course the tendency
for errors due to garbles, to upset the statistics. This has prevented
widespread psi setting (by machines) of P5twoback limitation traffic. Whenever
attempts are made, however, the "spanning device" is used - this is
a device on Colossus that permits the operator to set up a beginning
position and an ending position on his "spanning dials," and Colossus will
look at the material between these two positions only. Spans for
runs described are usually 800 letters in length.
The spanning device was put on Colossus for just these runs.
But it has been used constantly ever since for the other runs
(chi setting, etc.) and P5twoback runs are seldom made.
Crib runs and key-breaking runs are also done in Newmanry,
but they will be discussed later.