COPElAND | 149
The twelve wheels
The Tunny machine produced the key-stream by adding together two other letter streams,
called by Bletchley Park the ‘psi-stream’ and the ‘chi-stream’, from the Greek letters psi (ψ) and
chi (χ). The twelve wheels produced the psi-stream and the chi-stream; in fact, the wheels were
divided into three groups, five ‘psi-wheels’, five ‘chi-wheels’, and two ‘motor wheels’ (Fig. 14.3).
Each wheel had different numbers of cams (sometimes called ‘pins’) arranged evenly around
its circumference (the numbers varying from twenty-three to sixty-one). The function of the
cam was to push a switch as it passed it, so that a stream of electrical pulses was generated as
adder
V = R + H0
0
1
0
10
0
1
1
101111psi wheels (5)motor
wheels (2) chi wheels (5)(^01010) adder
R = P + M
type her
e
0
0
1
1
1
ciphertext
to radio VR
P
H M
figure 14.3 How the Tunny machine encrypts a letter.
Jack Copeland and Dustin Parry. All rights reserved.