The thin double walled structure of 'Nimrod's' Vacuum chamber! is such that the pressure differential between inner and outer chambers must be small at all times. Electrically operated valves at the ends of each vacuum chamber safeguard this condition. Beyond this, the need was felt for a parallel device of purely meohanical operation, to be operated by the pressure differential alone. This account shows the pre¬liminary thoughts and evolution of a prototype device. The characteristics of the prototype are given together with comments on the operation of several production assemblies.
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