Sometimes the most obscure and almost unrecognisable objects appear, and with all the interest in Christopher Nolan’s epic docu-thriller Oppenheimer,  I decided to put pen to paper.

Some years ago I was perusing an online auction and came across an instrument I recognised as a barometer or altimeter made by Negretti & Zambra. Puzzling, though, was the scale, calibrated in Millibars with a range from 300Mb – 100Mb – or, in altimetric terms, 30,000ft to 50,000ft! To add to that, it was marked “Port Static” on a piece of tape stuck on the dial.

Anyway, as this was an instrument of considerable technical excellence and appeared in good condition I bought it. No sooner had I paid for it than the seller contacted me and in a rather tentative manner said “Thanks for your order. Would you like another? I found two.” My reply was quick and simple: “Yes!”

Some days passed, and both instruments were now safely in my hands. Immediately I could see that they were consecutively numbered 114165 and 114166, which was in itself, quite extraordinary, and added to that, the second was marked in the same way as the first, though this time with “Starboard Static” on the tape.

When I find something new and interesting, the temptation to look inside is all consuming, so I set to work. The bezels unscrew the glass making an air tight seal with the case by means of a flat rubber seal. The bezel was easy enough to remove unscrew but the glass, which was seated in a rebate to create an air tight seal with a rubber flange below, was not moving. This is where a little guile is required, and so progressively the glass lifted, the hand was removed, the dial unscrewed, and below was the movement, of a pattern that frankly I don’t think has really been bettered for this type of equipment – exquisitely executed, a proper piece of scientific equipment of the highest order.

The movement pattern, in very simple terms, comprises two pressure sensing cells of a highly developed design acting in opposition about a common fulcrum, a short length of spring steel. Friction is therefore reduced to the barest minimum. The movement constructed between twin plates with thermal expansion slots. Calibration is via a removable plug at the rear below which is a screw that, when turned, causes by rack and pinion the whole movement to rotate within the case so moving the pointer up and down the dial, an idea originally patented way back in 1887 by Whiteside-Cook. The benefit of this means of calibration is that the precise pre-set parameters within the movement remain unaffected.

The outer case of cast and machined bronze is lacquered, and designed as air tight, the extending tubes located to the top and left of the instrument, as viewed from the front, intended to be plumbed into a pneumatic circuit. Thus any pressure value fed down from that circuit is translated faithfully to the dial.

Having serviced and conserved both instruments, the big question remained: what on earth could they have been used for?

The first major clue of course was in the tape stuck to the glass. Static ports are only found in aircraft – basically a simple means by which the outside ambient air pressure may be ascertained, generally connected to altimeters. It is a parallel system to the Pitot circuit where, either individually or together, these systems feed pressure to flight instruments – altimeters, airspeed indicators and variometers. Since the determination of altitude is a critical value in flight, aircraft became fitted with two autonomous circuits, Port and Starboard, which became mandatory after the blockage of the Static port was found to be contributory factor in a number of accidents – two circuits meant comparison of two altimeters, thus if one system had failed it would become instantly evident to the flight crew.

My first thoughts were that these might be flight test and development instruments possibly for Canberra, Vulcan, Victor or Valiant, but the problem with that theory was that the instruments, I can be sure, were built in 1956/7, this by serial number analysis from other dated instruments in the Archive.

Let’s go back to the scale, 300 – 100Mb. At ground level there is no reading – in fact the needles won’t start to lift below 30,000ft, which is about 300Mb – and the top end of the scale, 100Mb, equates to about 50,000ft. These instruments were specifically made to interpret altitude with great accuracy within this very high range, 30,000’ – 50,000’. There still remained the possibility that they were flight test instruments, but in discussing this with those that know, this became more and more unlikely.

Until one day I was on the phone talking to a contact with hands on experience of the V Force, and at the time embroiled in keeping the last active Vulcan XH588 in the air. I had sent him a photograph of the two altimeters ahead of our call. “Those are not what you think they are” he said. “What are they then?” I was bursting with curiosity. There was a pause, then “Grapple.” As I tried to remember what Grapple was, the voice at the other end of the phone piped up: “Britain’s first nuclear tests.”

After I put the phone down and reflected on the conversation, some quick research confirmed two vital pieces of information: the bombs were dropped from 45,000ft and the date of the first Grapple test was 15 May 1957. These facts absolutely fitted with what I knew for certain about the instruments. I needed a little more though, as I had not found any evidence or even suggestion as for what purpose these instruments may have been intended, having really extensively trawled the net, and talking to long retired airforce pilots and ground crew.

My contact at Vulcan had little more to add as, of course, the bomb was actually dropped by XD818, a Vickers Valiant. However, a long dialogue with someone that probably knows more about Valiants and Operation Grapple than anyone alive concluded that these instruments could well have been fitted to XD818 and were probably sited in the bomb aimer’s position – bombs were dropped manually from this position using a visual bomb sight, and this location already had a full port and starboard Static system plumbed in to operate the visual bombing computer.

So why use mechanical altimeters when there were radio altimeters on board? Well that’s simple – at detonation, a nuclear bomb releases a strong electromagnetic pulse, which might have a very negative effect on such a system. The other major reason, and perhaps more importantly, the drop altitude had to be very precise to ensure detonation at the correct height to reduce the risk of fallout, and radio altimeters of the time were not known for either reliability or accuracy. A very accurate, reliable mechanical altimeter had to be the answer.

Why is the scale divided into Millibars? That’s because mechanical altimeters, in order to accurately reflect altitude, have to be adjusted to prevailing barometric pressure. A ground level up-to-date barometric reading would have been transmitted to the aircraft and, from that information, altitude over the target could be accurately computed.

Subsequent conversations with surviving employees of Negretti & Zambra confirmed the rather hand-to-mouth system the company operated with the MOD. Orders would come in for “specials”, and the N&Z team would set out to produce what was required. These two special instruments have one other very unusual attribute, black dials, the material unusually being some sort of composite, all other dials on similar pattern instruments so far seen are brass and silvered.

It is interesting to note that the above arrangement of the movement and design of components is found only in this generation of instruments, to include the very rare Micro or Precision Barograph detailed elsewhere on this site. Negretti & Zambra has an almost fabled reputation based on much earlier production, namely that from the Victorian and Edwardian era. In point of fact, that reputation is rather misplaced, and, although N&Z were certainly innovators in terms of the invention of pocket and watch-sized aneroids in the early 1860s, much of the production in the later 19th century was fairly mainstream. The great acclaim that Negretti & Zambra has should really be based on the instruments they designed and manufactured in the early to mid 20th century, instruments produced at the time of new materials, new science, and new technology, and perhaps unparalleled in its quality in making.

Of course, almost all the brave men that flew such aircraft sitting for days on QRA (quick reaction alert) at the height of the Cold War are now passed, and the opportunities for first hand accounts gone – slow painstaking research is the only way to run down the facts. Operation Grapple was a very closely guarded secret, aspects of which still remain classified to this day. Procurement of instruments and equipment was conducted very much on a need-to-know basis, so the order for these instruments from Negretti & Zambra would have been made with a general specification only. All of this makes finding the incontrovertible proof for the history of these instruments very difficult.

Grapple Altimeters_3a
Grapple Altimeters_2a

Valiant XD818 during Operation Grapple (photo: VForce HQ Archives)