The US 6812 Division Bombe Report Eastcote 1944 Page 86

Tony Sale's Codes and Ciphers

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- 71 - TOP SECRET-T
cam closes its contacts, all "intermediate" drums on the bombe are advanced
and each time a "hot spot" on the back of the "indicator intermediate" drum
makes contact all "slow" drums advance.

The seven multi relays are of a rugged multiple contact type, each

relay carrying 12 sets of transfer contacts and measuring 4 3/4 inches
long, 4 3/8 inches high and 2 3/8 inches wide. The mounting of the core and
the armature action resembles somewhat the "standard general purpose"
relay. The horizontally mounted core attracts an L-shaped armature which,
by virtue of being hinges at the front end of the relay, pushes an
auxiliary armature upward. This auxiliary armature is hinged at the rear of
the relay and operates the moving springs of the 12 sets of contacts
through a large insulating stud mounted on the front end of this auxiliary
armature. This relay is designed to operate in .012 to .020 seconds and
release in .006 to .012 seconds.

The 3 sets of 26 indicating unit coils each are mounted in an

assembly known as the No. 513 80 column International Business Machine
Corporation Comparing Unit. Two or the columns are not used. These are the
27th and 54th columns. The energizing of one of those magnets functions a
linkage which causes a pointer to give an indication on a horizontal scale.
A bail lever with one pair of "make" contacts and one set of "break"
contacts is also part of the indicating unit. The bail lever is operated to
test the indication to insure that it is not faulty. The "make" contact
operates a relay which breaks the "sensing" circuit and requires the
"sensing" relay to reoperate when the lever is released. The "break"
contact opens the clutch magnet circuit while the lever is operated.

The "heavy duty" relay is of identical construction with the multi

relays except that it has only two sets of "break" contacts instead of 12
sets of contacts.

The circuit breakers consist of heavy duty normally closed contacts

which are opened at carefully timed intervals to insure that the "sensing
circuit" is broken at these circuit breakers instead of at the commutators
associated with the enigma drums. These contacts are actuated by a gear
shaped plastic cam on which rides a roller attached to a pivoted arm
carrying one of the pair of contacts. The pivoted arm is held against the
toothed cam by a flat spring.

The cams for controlling the time of closing the electrical circuits

resemble the circuit breakers except that the contacts are of lighter
construction and the method of actuating the moving contact is slightly
different. In this case one end of the moving spring is fixed and the other
end rides on the surface of a cam whose contour varies from a low spot
gradually to a high point and then abruptly drops to the low point. The
contacts function as either "make" or "break" contacts dependent on which
of the pair of springs bears on the cam.

The control commutator is made up of a disc of insulating material 1

1/2 inches in diameter and 3/8 inches thick, 2/3 or whose periphery is
encased in a metal tire on which ride two spring contacts. The commutator
merely serves to close a circuit between the springs while they ride on the
metal tire.
MECHANISM OF BOMBE.

A mechanism consisting of drive motor, belt drive, clutch, cams (both

electrical and mechanical), index wheel, circuit breakers, commutators,
carry elements and drums provide the moving parts of the bombe, exclusive
of the relays, and operate under control of the electrical circuit just
described.

The motor is a Thomson Houston Shunt Wound Direct Current 220 Volt

Motor rated at 0.75 horsepower and drawing 3.8 amperes when running at 800
rpm or 3.42 amperes at 1000 rpm. The belt drive utilizes an ordinary V belt
running from a pulley of 2 inch diameter on the motor to a pulley of 6 inch
diameter on the clutch. The clutch is of the pawl and ratchet type, and is
magnetically controlled. The power is supplied through a shaft connected by
belt drive to the motor. The driving ratchet is pinned to this shaft, and
when the pawl is engaged, transmits power through the driving pawl arm to
the main gearing which drives the vertical drive shaft. Each pair of gears
uses one fiber gear in this main gearing with the intent that in case of
some mechanical bind throughout the mechanism the teeth on the fiber gear
will


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