Declassified Dollis Hill War Diary made public

One of the difficulties in showcasing the history of espionage is that many important documents remain classified – and thus our picture of these shadowy areas of history remains incomplete and patchy.

We were thrilled, therefore, to see that BT Heritage and Archives has now published the ‘Dollis Hill War Diary’ – a historical record of the involvement of the General Post Office’s (GPO – as BT was formerly known) Dollis Hill Research Station and its involvement in the Second World War. The GPO was, at the time, a government department which had something close to a monopoly on the UK’s telecommunication services – telephone and telegraph services. At its Dollis Hill Research Station, all kinds of new telecoms equipment and services was trialled, and it became a natural ally of the war effort as a result.

The story of Colossus, the world’s first digital electronic computer, which helped Bletchley Park intercept encrypted enemy communication signals, has only been revealed in the past 40 years – and it was developed by the team at Dollis Hill.

The Dollis Hill Research Branch also did a considerable amount of work for the CSDIC at Trent Park (as well as Latimer House and Wilton Park), helping them develop, source and install the microphones and recording equipment which made these centres, and the work of the Secret Listeners, possible.

In December 1939, the diary reveals:

On 6 November a request was received for co-operation in a large installation at Trent Park, Cockfosters, and this was completed by 13 December although the building had not been ready for wiring until 4 November.

In six room, to be used for interrogation purposes, the microphone is installed inside the ordinary extension telephone which is fitted. In another eight rooms the microphones had to be so concealed as to be undetectable to the occupants even should they make a careful search. IN six of these microphones were installed above perforated guards in ceilings, the whole of each ceiling being papered. After some laboratory experiments a method of mounting a microphone in an ordinary electric ceiling rose in such a way that the rose could retain its normal function and the microphone remain invisible if the rose were opened was devised.

The office in charge expressed great satisfaction with both the appearance and working of the equipment in the two rooms entirely fitted up by Dollis Hill.

And on July 1940, it was recorded that:

Such good results had been obtained from the original installation at Cockfosters Camp that an increase in the number of rooms fitted with microphones was desired. It was decided that the P.O. method of concealing the microphones should be adopted since it was simpler than the R.C.A. method and less likely to be discovered (one of the latter had actually been found by a “guest”).

The diary, which was classified for reasons of national security up until now, will be an invaluable source for historians, revealing a little more of the history of espionage, communications and technology developed and used in the Second World War. The full diary is now available to view online via BT’s website.