11 - Bombe Speed
As already stated, the minimum time for a full run (one without stops) on both a 39 point and a 30 point machine is 26 x 26/65 = 10.4 minutes on the assumption that the carry rate was the same in both types of machine. In some cases it could take the WRNS operators up to 30 minutes to plug up and equip a machine with drums and then check these.
Almost all the early 73 three-wheel machines fitted with BTM differential sense relays were 39-point. Of the second batch of 69 three-wheel Bombes fitted with Siemens sense relays, 11 were 39-point. The rest (57) were 30-point. The rebuild machine gearing is the same as the 11 machines in the second batch.
The top drums rotate at 65 x 1.5 = 97.5 rpm = 1.625 revs per second. Sensing takes place between each letter of the top drums at 1.625 x 26 = 42.25 times per second. Time between each of the 26 commutator segments on the top drums is 24mS. All segments are sensed in just under 0.62 seconds. Then the mechanical phase, where the carries occur, takes a further 0.31 seconds.
Sensing 26 positions (once per carry): 0.92 seconds
Test all 17,576 positions: 0.92seconds x 26 x 26 = 10.4 minutes without a stop.
(c) John Jackson
