The clutch and relays coils used are part of the BTM family of relays including the Siemens High Speed versions that carried on after W.W.II. It is possible that BTM moved into the Siemens type as a direct result of Bombe development but obviously this could not be recognised at the time for security reasons.
We have collected together a good selection of suitable relay types but with a few shortages that are listed elsewhere on this WEB site. However as luck would have it we have very few coils where the resistance value matches our requirements.
As part of this whole coil winding programme we also had to wind insulated resistance wire onto recovered GPO formers. These had to be non-inductive so entailed reversing the winding half way through the operation. Alan Wray completed this work on 104 formers.
Therefore we had to embark on a coil winding programme. Dennis Monk arranged for the supply of soft iron, machined it as appropriate and the arranged for the cores to be professionally annealed.
Alan Wray then took over the actual coil winding and with the help of Peter Porter refurbished and adapted an old commercial coil winding machine. Some of the wire we need to use is as small as 39 s.w.g. (approximately 4 thou in diameter) and therefore has to be very carefully handled to avoid breaking.
The above coils are used in the clutch (2 off), Heavy Duty Relays (2 off) and the Multi Relays (7 off). They are the largest type used in the Bombe.
Above are examples of BTM ‘A’ type coils. Approximately 40 of these are used around the Bombe. Most are differentially wound as are the examples above. Two different windings are wound one over the other on the same core. This adds to the difficulty in obtaining satisfactory termination of the very thin winding wire.
Above are special coils used in two High Speed BTM relays. Not to be confused with the Siemens High Speed Relays below. They are similar to the ‘A’ types above but shorter. Each of the two relays has two of these coils fitted. Very similar versions are used in the carry mechanisms.
Above are examples of rewound coils used in the Siemens type High Speed Relays. In this case the formers have been recovered from existing relays. Two coils are required in each sense relay making a total requirement of 208.
To see more about the coil winding machine press