The US 6812 Division Bombe Report Eastcote 1944 Page 88

Tony Sale's Codes and Ciphers

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- 73 - TOP SECRET-T
operated through gearing so designed that when the carry mechanism is
effective the intermediate drums are advanced one position for each
revolution of the fast drums; and similarly the slow drums are advanced one
position for each revolution of the intermediate drums.

When the carry magnets are energized the carry hooks are engaged with

the carry cams and cause the carry bars to be pulled horizontally to an
extreme position and then restored to the normal position. The carry hook
is attached to and hinged on the carry bar. This reciprocating action of
the carry bar is translated into a rotary motion of the drum by means of a
pawl and ratchet, which also serves to move the drum one position. The
shafts for 12 drums are advanced with each stroke of the carry bar.

The scanners, Set-up and Cancel commutators, are driven through worm

gearing from the vertical drive shaft. The bearings for these gears are
simple bearings mounted in the vertical guard plate. These bearings are
adequate since there is no great torque transmitted through these gears.
The commutators consist of insulating discs in which are set 4 concentric
rings of 26 contacts each. A brush arm with 4 brushes sweeps over the
contacts on each commutator. A time relation is established between the
sweep arms of the 2 commutators by adjusting them so that the brushes of
the Cancel commutator make a given set of terminals before the brushes of
the Set-up commutator makes it's corresponding set of terminals.

Eleven electrical cams mounted on one shaft are also driven through

worm gearing from the main shaft also. These cams serve both to open and
close electrical circuits at definite intervals with respect to the index.
There are two types of cams, - "make" cams and "break" cams. The "make" cam
closes at a definite point, remains closed for a period determined by the
cam contour and then opens at some point within limits depending upon the
tension of the springs. For instance cam 6 is a "make" cam and closes
precisely at 30 1/2 and opens at about 17 1/2 to 18. The "break" cam opens
at a definite point, remains open for a period determined by the cam
contour and then closes at some point within limits. Thus cam 7 is a
"break" cam and opens precisely at 26 1/2 and closes at about i3 1/2 to 14.
The cam is made of fiber with a contour of the logarithmic spiral type and
the determination of whether it is to be a "make" or "break" cam is
dependent upon which one of its pair of springs rides on the cam.

The indicating drums and the electrical carry cams are driven by the

same horizontal drive shaft and carry bars that drive the regular drums of
the intermediate bank.

The control cam is a ceramic type cam with a metal tire encasing 2/3

of its periphery and is driven through gearing from the vertical drive
shaft. Two brushes bear on its surface and form a closed circuit when
resting on the metal tire.

Six circuit breakers on a horizontal shaft are driven through gearing

from the vertical drive shaft. These breakers consist of a fixed contact
and a moveable contact each. The moveable contact is mounted on an arm
pivoted in the middle with the contact on one end and the other having a
roller. The roller rides under spring tension on a plastic cam whose
periphery has a series of 26 cupped indentations. Two of these breakers are
used to close the circuit which supplies current to the sensing circuit at
exactly 1/4 of a position before the index point. The remaining four are
designed to break the circuit at exactly 1/4 position after the index
point. With these adjustments the current flowing in the sensing circuit is
made and broken on these circuit breakers and not at the commutators or
relays.

The bombe is equipped with an index to provide a reference against

which all timing adjustments are made. The index is a dial over which
sweeps an indicator driven through gearing from the vertical drive shaft.
One complete revolution of the pointer covers one sensing cycle and one
carry cycle. The complete revolution is divided into 30 equal positions, 26
of which are devoted to the sensing cycle and 4 to the carry cycle.


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