The Colossus Rebuild Project by Tony Sale



Colossus Reborn

The rear racks of Colossus

Looking along the rear racks of Colossus towards the W rack, the one holding all the GT1C gas filled Thyratrons which make up the ring circuits representing the wheels on the Lorenz cipher machine.

The Thyratrons are gas filled triode electronic valves. Because of the inert gas in the valve, an electronic arc is struck between the anode and the cathode when the anode has a positive voltage with respect to the cathode and the grid voltage is raised towards the cathode voltage to allow current to flow. Once this arc is struck the grid voltage has no effect on it. The arc can only be quenched by driving the anode negative with respect to the cathode. The Thyratron thus "remembers" it has been struck and acts as a one bit store. This is very economical on valves, a one bit store in other early computers required two ordinary valves.



This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park Heritage Society.