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Instruments and equipment.

Please note that none of the equipment on this page is kept at the Post but is displayed on our Open Days.

This page shows some of the major equipment in use by the Royal Observer Corps in Posts up until Stand down of the Posts on 30th September 1991, it is not a comprehensive list but will give some background.

Cold War era.

Bomb Power Indicator

This is our copy BPI, kindly loaned by Al McCann of Portadown Post in Northern Ireland. Real BPI’s are very hard to obtain and command a very high price. Inside the instrument is a brass bellows which would have been expanded by the shock wave from a nuclear explosion, this would have moved the needle and a reading would be obtained. Using the Peak overpressure reading and the distance from the explosion, the triangulation team at Group headquarters would have been able to determine the power of the bomb. To add to the realism of a visit to 51 Post, the copy BPI contains electronic circuitry to simulate random readings at random time intervals.

Ground Zero Indicator

The Ground Zero Indicator, more commonly known as a GZI, is a very simple instrument which allowed the triangulation team at Group headquarters to find the precise location of a nuclear burst. The GZI comprises 4 pinhole cameras which are orientated North, East, South and West. In the event of a nuclear burst, the flash from the explosion would create an image on the photographic paper inside. Each of the 4 papers was contained in a transparent “cassette” which was marked with degrees of elevation and direction. With the GZI information from at least 2 Posts the exact location of the burst and it’s height above the ground could be calculated. The height above ground and the power of the bomb would determine if the bomb was a ground burst which would create lots of fallout, or an air burst producing very little. Our GZI has been very kindly loaned by Gavin Saxby.

Fixed Survey Meter (PDRM82F)

Used to measure radioactivity above ground, the sensor (left of photo) of the PDRM82F was mounted on a telescopic rod and pushed up a steel pipe into a plastic dome. A cable led from the sensor to the digital display positioned in front of the Observers on duty. Also used was the PDRM82 (no F) which had the sensor mounted internally and was therefore to be used as a portable instrument.

AD8010 Teletalk

Communications between Group control and each cluster of 3 or 4 Posts was by a dedicated telephone line. In the post this was connected to the Teletalk, a loudspeaker telephone. Powered over the line from the exchange, opening the lid released the power button turning the unit on. Pressing the call button rang the Post Display Plotter position on the balcony in Control, the third button is pressed to talk and released to receive. In the Post you could hear both the Plotter and the other Posts in your cluster. Thanks to modern technology the Teletalk at 51 Post works again even though the phone lines are long gone. As Carlisle Control has also gone, we’re now connected to 28 Group Control at Craigiebarns, Dundee. If there’s no-one to talk to the line plays recordings of old ROC exercises.

AD3460 Teletalk

The earlier Teletalk was not powered by the telephone line voltage but by an internal 6V battery, it also contained a 67.5V battery which was connected to the line to ring the Post Display Plotters position at Control. As of yet we haven’t sourced a genuine AD3460 but we do have a replica which houses a 51 Post created “Telefake” circuit board which means it can work on the new comms system. The “Telefake” pcb’s have been made available to other restorers.

WB1401 Carrier Receiver

Warnings of attack would be passed to 250 Carrier Control Points (CCP’s) from either Strike Command or the backup location at Goosnargh near Preston. The CCP’s, which were in major police stations, broadcast to around 19,000 warning points in places like Police stations, Post Offices, Town Halls, Churches, private houses and of course ROC Posts. Handel as the system was called used a high frequency inaudible signal superimposed on the speaking clock network, the warning points needed a special receiver to hear the transmissions. Commonly known as WB1400 the system used two models of receiver the WB1400 which was for general use and the WB1401 used by the United Kingdom Warning and Monitoring Organisation (UKWMO) of which the ROC was part. Transmissions were preceded by a number of special “wake up ” tones, the WB1400 needed 8 to wake up, the WB1401 only 4, this way messages could be sent to the UKWMO for training without panicking the general population. ROC Posts used a metal, waterproof loudspeaker, other locations used a plastic loudspeaker. Both types of speaker carried a Warning District number which allowed messages to be targeted to certain locations. 51 Post Threlkeld was in Carlisle 34 Warning District.

WB400 Carrier Receiver

The WB1401 carrier receiver was a replacement for the WB400 and differed in several ways. Perhaps the biggest change as far as Observers were concerned was that the WB400 once turned on produced a continuous pip sound 90 times per minute unlike the WB1400 series which remained silent unless either the test button was pressed or the receiver detected the 650Hz wake up tones. The WB400 was a single unit containing both receiver and speaker in one housing as opposed to the separate WB1400 series units. As the WB400 was active as soon as switched on, there was no selective calling facility. Unlike the later WB1400 which housed a rechargeable battery which charged from the telephone line, the WB400 used the same type of 6V battery as the AD3460 Teletalk, therefore a stock of batteries had to be held wherever a receiver was installed. The 51 Post collection now includes a genuine WB400 which can be heard working when on display. Anyone attempting to get a WB400 working should note that the wiring colours for the battery are normally, black +6V and purple (connected to the switch) negative.


Upon receiving warning of attack over the Carrier Receiver, one Observer would leave the post to sound the siren and warn the general public in the vicinity. It is reported that the hand wound siren can be heard up to a distance of about a mile. The siren would also be used to sound the all clear. The ROC used two models of siren, the Secomak (Service Electric Co.) and the Carter, both similar designs other than the stand, the version in the picture is the Secomak.

Fallout Warning Maroons

Most definitely on our wish list is a Maroon trainer kit. Manufactured by Pains-Wessex, the Maroons would be fired to warn the public of the imminent arrival of fallout. Above is a picture of the instructions which nicely explains their use. Click on the picture for a larger view.

Petrol Electric Set

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Lighting in the Post was powered by a 12V battery which was alleged to be able to run the light for 110 hours, in practice it was meant to be charged every alternate day for 2 hours using a small generator officially called the PE Set, PE standing for Petrol Electric. The original PE Set was made by either Swan in Banbury or Morrison in Southampton and used a Villiers 507-H1 engine. Towards the Stand down of the Corps Yamaha EF1000 generators were starting to be issued as a replacement, we are lucky in having an EF1000 in addition to our Swan PE set.

Aircraft Spotting role.

Post Instrument

Post instrument.
The Post instrument was originally designed by R B Pullin & Co. in 1934 and in 1940 the instrument was improved with the addition of a height corrector which was devised by an Observer Micklethwait. This adaptation has led to much confusion over the years with many incorrectly calling the Post Instrument a Micklethwait. In use the height of aircraft, either estimated or confirmed, was set on a vertical scale, when the aircraft was then located in the view through the eyepiece its position over the ground was displayed on the circular map table. If the aircraft could be seen by 2 Posts or was directly above a Post then the Micklethwait attachment could be used to accurately determine its height.


Posts were issued with low powered fixed focus binoculars to enable aircraft to be identified, the following information comes from extensive research but cannot be guaranteed 100% accurate. Binoculars issued in the WW2 era were predominantly Air Ministry specification 6E/293 manufactured either by Wray, Charles Franks or Watson & Baker. Others were used in smaller numbers mainly ex Canadian Naval binoculars, model and manufacturer unknown. From the 1947 Stand-To onwards, it appears that the wartime issue had been collected in at Stand-Down in 1945 and although there were some 6E/293’s re-issued the majority were 6E/383, 6E/320 or 106E/8. I can however find no reference to the latter and believe the inventory this number comes from is incorrect, (perhaps mis-transcribed to the inventory). As is the norm with MOD purchases, there could be different manufacturers of each type number. Binoculars to 6E/383 specification are known to be have made by Wray, Barr and Stroud, Watson-Baker and Ross (REL).


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