- 74 - TOP SECRET-T
THE CHECKING MACHINE. GENERAL DESCRIPTION.
The checking machine could be considered as an enigma machine as it
has the components to perform encoding and decoding. However its
construction does not facilitate encoding of text. The parts used are
similar to those used in bombe construction. The checking machine is so
arranged as to facilitate the testing of bombe solutions for their
cryptographic worthiness. The checking machine has positions for four
wheels, or drums, so that it can be used for any type of enigma work; Army,
Navy or Air Force, and is built by the British Tabulating Machine Company
Ltd. of Letchworth England.
INSTALLATION AND PREPARATION FOR USE. As received from the factory
the machine is mounted on a wooden stand so constructed as to store the
eight drums within easy reach of the operator. Drum separation cleats on
the shelves are removed as it was found that their presence caused drum
clip springs to become damaged.
The stecker plugboard mounted on a clip on the side of the machine
stand is not necessary to the testing of the bombe solutions and so is
removed and the lip cut off. The circuit through this plugboard is then
wired straight through without
crossovers. This step was found to be of value in making the checking
machine so that it required less repair time.
In order to preserve color coding on the drums each shell is knurled
and the knurl filled with paint. The normal point on the shell, marked with
a simple pin point as it comes from the factory, is replaced by a screw
head for permanence and ease in use. The machine operates from regular 50
cycle 220 volt supply mains and consumes a negligible amount of current.
DESCRIPTION OF DRUMS. Checking machine drums are similar in
construction to the bombe drum. They differ in two respects; the
ringstellung and the brush construction. The ringstellung in the checking
drum is variable whereas it is fixed on the bombe drum. Provided with a
ball bearing positioning device in the hub the ringstellung comes to rest
with a letter exactly opposite the normal (screw) point. The checking
drum brush is made from phosphor bronze in the shape of a leaf spring and
having a small upturned U at the end. The phosphor bronze material gives
constant springiness necessary for positive contact with the commutator in
the operation of the drum. The U portion at the end of the brush contacts
the brass segments of the drum commutator. This upturned U at the tip of
the brush allows the checking drum to be turned in either direction without
damaging the brush.
DESCRIPTION OF PARTS. The principal parts used in a checking machine
are as follows:
1. ON * OFF Switch.
2. Step-down transformer
3. 26 Letter keys (one for each letter of the alphabet)
4. Associated with the keys are twenty-six corresponding lamps
each lighting a letter of the alphabet.
5. Drums and drum commutator mountings.
6. The U/W (unclewalter) turn around plug and socket.
The ON - OFF switch needs no explanation. The purpose of the step
down transformer is to reduce the input volts of 220 to 75 for the lamps.
The letter keys are actually a transfer switch. When the key is in the
normal position; i.e. -not operated, a circuit is completed through the
associated lamp. When the key is operated the associated lamp circuit is
broken and battery potential is placed on the drum combination. The
provision for breaking the lamp, circuit is necessary due to the fact that
the German enigma machine can encode no letter as itself. The U/W plug
provides the current "turn round" point as does the identically named part
in the German enigma machine. There are two types of U/W plugs; one which
is wired straight without crossovers and which corresponds in conjunction
with the fourth drum to the "B" U/W in use at the end of hostilities. The
other type of plug is the eel plug (also used in the bombe on certain
cases) and which is made pluggable so as to correspond with the pluggable
U/W (unkewaaltz) introduced by the Germans in July 1944.