its purpose and operation
by Tony Sale| Back to Fish Index |
Codes and Ciphers
|The mathematician Max Newman now came on the scene. He thought that it would be possible to automate some parts of the process for finding the settings used for each message. He approached TRE at Malvern to design an electronic machine to implement the double-delta method of finding wheel start positions which Bill Tutte had devised. The machine was built at Dollis Hill and was known as Heath Robinson after the cartoonist designer of fantastic machines.|
When Wynn-Williams was asked to produce electronic circuits to implement the
double delta algorithm he chose to use a phase modulated carrier from a master oscillator
at 25kc/s to perform the
He decided to use 0 and 180 degrees of phase to represent 0 and 1. The elegance of this is that if a "1" causes 180 degrees phase shift, then another 1 returns the phase to zero and thus this implements an XOR function (0 + 0 = 0, 1 + 1 = 0, 0 + 1 = 1, 1 + 0 = 1).
|The 180 degrees phase shift was achieved via a diode bridge circuit and a balanced transformer. The biasing of the bridge, + - 10 volts, determined whether the input carrier went straight through (no phase change) of shifted 180 degrees. A triode valve amplifier was included with each bridge circuit to compensate for the losses in the bridge and to give unity gain from input to output.|
|The output phase at the end of a series of logic circuits was compared with the phase input to the logic circuits in a detector circuit. This gave a voltage output of nearly zero if the input and output are in anti phase or some, much larger, positive voltage if they were in phase. The output voltage from the detector was sampled by a pulse derived from the sprocket hole signal from the tape reader. The result of this sampling, either a pulse if the detector output was positive, or no pulse if the output was zero was then passed to the decade counters to accumulate a count down the whole length of the tapes.|
Heath Robinson was delivered to Bletchley Park in June 1943 and was first installed in Hut 11 which had been the original Bombe room for Turing Bombes, the machines used to break Enigma.
Harry Fensom and Alan Bruce were the two GPO maintenance engineers assigned to Heath Robinson. Two WRNS(Womens Royal Naval Service) ladies at a time were the operators and Jack Good and Donald Michie were the code breakers.
The first problem was teleprinter tape preparation. At least 2000 characters of cipher text was required, joined end to end to make a continuous loop. Then a similar length of Chi wheel patterns had to be punched up and arranged to be just one character longer than the cipher tape. This was to automatically change the relative wheel patterns by one position after each complete run through the tapes.
Then it was found that the optical readers in the Bedstead gave errors if a long stretch of adjacent holes or no holes occurred on the tapes. This meant adjustments to both texts to compensate for this.
A major problem was keeping the two tapes in synchronism at over 1000 characters per second. Originally the sprocket drive cogs were motorised but this proved impossible to sustain without tearing the tapes and a friction drive was used from the paper tape pulleys with the sprocket shaft just idling to keep synchronisation. This proved to be better but there was still a problem with tape stretching in the distance between the sprocket cogs and the optical reader aperture.
Heath Robinson worked well enough to show that Max Newman's concept was correct. Newman then went to Dollis Hill where he was put in touch with Tommy Flowers, the brilliant Post Office electronics engineer. Flowers went on to design and build Colossus to meet Max Newman's requirements for a machine to speed up the breaking of the Lorenz cipher.
|This page is created by Tony Sale
(email@example.com) the original curator of the Bletchley Park Museum