Can Fuse Settings Be Related to Time?

[Rockdoc]

One of the AA Sections in Salonika often gives the settings for the fuzes on its shells in its Diary, usually recorded as "at fuzes 18, 20 and 22." Having done some very rough calculations, these seem to be about the right flight-times for the shell to reach an approaching aircraft in seconds. Can anyone confirm this interpretation?

I was going to add "or shoot it down" to the last sentence but thought I'd save everyone the indignity.......!

Keith

[Old Tom]

Sounds correct to me. I don't think it could be any other parameter (don't often use that term, hope it's right here). If it were a distance it would depend on the angle of elevation of the gun when fired. In any case I think this was well before any sort of predictor and the fuse setting would be a 'guesstimate' and I wonder if the fuses were those in general use rather than a special for anti aircraft use.

Old Tom

[healdav]

I think the first shells were set by time. Later they were set by height - with members of the gun crew estimating the height. Presumably this also was a set number of seconds per X feet.

Only during WW2 were proximity fuses introduced (invented) - Churchill gives a lovely picture of members of the War Cabinet crowding around and waving their hands in front of a prototype to see a little light flashing.

[Rockdoc]

I believe the fuzes used were standard but I'm not certain. They were early on and there was a lot of trouble with them, all basically down to the use of a burning ring of powder to give a timing effect. The composition of the powder was modified so that it would continue to burn at a consistent rate at higher altitudes, for example. As they modified AA guns and ammunition to increase muzzle velocity - the phrase suck it and see comes to mind with most developments of AA equipment in WW1 - they suddenly got a lot of duds, which was eventually traced to the fuze being spun too fast, causing the burning front to become separated from the rest of the powder. It took a lot of experimentation to work out the problem - they built a chamber that could be given a partial vacuum containing a turntable that rotated at variable speeds so they could simulate the shell's flight - and the remedy was to reduce the tightness of the scroll in the rifling so that the shell travelled faster but rotated at the 'normal' speed. (Ref History of the Ministry of Munitions Vol X)

Fuze setting was not estimated but it was a more involved calculation. Because the elevation of an AA gun can be much greater than for a field gun and because the shell explodes at height, the variation in shell speed with elevation after a given time period - due to the effect of gravity - became very important. AA guns were fitted with a 'fuze range dial' that was geared to the elevating system and directly gave the men the setting for a given elevation and range. You don't see many photos of AA guns with them fitted, though, and if the experiences of the Salonika-based Sections is anything to go by they were very fragile and were always stripping their teeth or bending shafts so it isn't hard to imagine them being taken off but if, as would appear to be the case with this Section, they'd found as time went by that the fuze settings they used didn't vary very much then why would they bother to keep the FRD working when it was a pain in the bum?

Healdev, they could estimate height without guessing. Two men, called rangetakers, were stationed about 1000 yards apart and each had a sight that could give a horizontal and a vertical angle. That would give the range and the altitude through triangulation. According to the Ministry of Munitions book, a set-up as I've described would allow them to estimate 12,000 feet to an error of +/- 300 feet. That's not fantastic accuracy but if the flight time was about 20 seconds for the shell the changes a pilot could make in that time make 300 ft seem trivial.

Keith

[ianjonesncl]

There are some times of flight and heights on wikipedia which may be of interest.

http://en.wikipedia.org/wiki/QF_12_pounder_12_cwt_AA_gun

The information is cited as originating from Brigadier NW Routledge, History of the Royal Regiment of Artillery. Anti-Aircraft Artillery, 1914-55.

This would seem to indicate that the height of burst was around 15,000 feet.

Would that make sense?

[Rockdoc]

That's a great table. Thanks for the link. Yes, it makes a lot of sense. In Salonika there was a mixture of 13-pdr 6-cwt and 13-pdr 9-cwt guns, the former having a muzzle velocity of about 1600 ft/s against the 2000 ft/s of the latter. Adding about 20% to the flight time for the 6-cwt gun would make 15000 ft achieved after about 27 seconds and that's a very long time! It's no wonder the Diaries get excited when they got a tight group of bursts around a plane, whichever nature of gun they were using.

No Diary lists a plane's altitude above 15000 feet. Anything more was simply written as "at great height" and if they fired at all under those conditions it would be what they called a "directing" shot to alert patrolling Allied planes rather than to attack the hostile plane. Directing shots were also used if planes were out of range of the guns.

Keith

[healdav]

Healdev, they could estimate height without guessing. Two men, called rangetakers, were stationed about 1000 yards apart and each had a sight that could give a horizontal and a vertical angle. That would give the range and the altitude through triangulation. According to the Ministry of Munitions book, a set-up as I've described would allow them to estimate 12,000 feet to an error of +/- 300 feet. That's not fantastic accuracy but if the flight time was about 20 seconds for the shell the changes a pilot could make in that time make 300 ft seem trivial.

Keith

You are correct. I was just trying to keep it short - I didn't say, guess, but estimated.

[Rockdoc]

Sorry. Crossed wires.

Keith

[MartinWills]

There is a little discourse on fuze types Here

I well remember Martin Middlebrook explaining the nature of the shrapnel shell and its fusing with the aid of several official drawings. One thing that impressed me at the time was the range of fuses, though Martin explained that for shrapnel shells the most common fuses were "T&P" or "Time & Percussion" detonating either after a given time (set on the dials on the fuse/nose cone) or on percussion - when the shell hit something.

[nigelfe]

Before proximity fuzes the only way of bursting a shell in the air was with a time fuze. Obviously the 'real' was based on the time of flight to the required point of burst. ToF is itself a function of range (slant range not height for AA), muzzle velocity and trajectory.

The Germans did have some mechnical time fuzes in WW1, at least I assume so because they invented them at the beginning of the century! However, AFAIK UK only used igniferous (ie powder burning) time fuzes in WW1.

The logical unit of measurement for a fuze setting is time of flight in seconds. However, British igniferous fuzes did not seem to use seconds. The scale appears to be arbitary, with a maximum value of 22 in most if not all cases in WW1. This of course does not matter because an arbitrary scale can still be related to the time of flight. The Range Tables give the fuze setting for range. However, in practiice the Fuze Indicator was used, with range as one input, because the FI (often called FBI) gives a fuze setting corrected for calibrated muzzle velocity and allows use of the Corrector which reflects previous shhots and hence current conditoins.

[Rockdoc]

The History of the Ministry of Munitions Volume X says that the Germans started to use mechanical fuzes late in the War and that he Entente had begun to reverse-engineer them but had not completed the work before the war ended.

I've no doubt there were tables for the various natures of AA gun but I'm not convinced they were the final answer on how to set the fuze. Height and range matter more in AA work than in 'conventional' gunnery because gravity affects the rate of decelleration of the shell depending on the elevation used, i.e. the flight time and flight-path shape vary continuously. The few images I've seen of the fuze range dials on AA guns, which were geared to the elevation (as was the sighting telescope for the same reason) shows the boundaries of the zones were curved. That said, as I wrote earlier, most photos do not show FRDs, which suggests they weren't considered vital in practice, all the more so when they seem to have failed quite frequently. It makes you wonder whether someone transcribed the figures from the FRD while it was working or the Sections were eventually provided with a supplementary table instead.

Keith

[Rockdoc]

Further to my last post, the Diary of 91st AA Section records, after an engagement interrupted by the appearance of two Nieuports:

19/10/1917: Pinion on fuze-range dial broken

How long the repair took isn't mentioned but it had been because:

28/10/1917: A Sub engaged hostile plane 'ALBATROS D.III' type, attacking balloon. Very hazy & visibility very low. Had to cease fire several times owing to balloon being in line of sight. Balloon descended in flames. Fired 32 rounds.

Clearly a badly-blotted copy-book but the next line records a classic bum-covering exercise:

Pinion of fuze range again broken, causing us to fire 'blind'.

So some Sections were definitely using them regularly - or used them as a convenient excuse......

Keith

[nigelfe]

Self-evidentally AA gunnery was primitive in WW1. AA fire was consider 'slanting' fire whereas field gunnery was horizontal fire. AA fire has always been based on the ascending trajectory, the aim being to put the shell where the a/c will be at the time of flight after firing.

As I understand it the AA fire control instruments were an identification telescope (for a/c recognition), an optical range finder (1 or 2 metre) and Wilson-Darby sights (all tripod mounted). I think the GPO may have used the data from these instruments to order a deflection to the guns, but it could be that he just estimated deflection. I'm assuming that the W-D sights were a very primitive predictor, but there were a lot of 'good ideas' developed for AA instruments to solve the 4D prediction problem.

For 13-pr 9cwt, the guns set the vertical and horizontal deflection on their sights and and tracked the target through their sights, this meant the gun was aimed ahead of the target. However, in elevation the sighting gear also used the Tangent Elevation provided by the Fuze-curve Plate (according to Routledge). However, I can't work out why the TE was needed as well as the deflection if the layer was tracking the target, logically the actual TE would be reflected on the F-c P, which it would combine with the slant range (from the rangefinder) to produce the fuze setting.

ToF for 13-pr 9 cwt at 55deg TE and 15000ft height was 22.1 secs. Given the slow moving targets its possible that the GPO ordered the fze setting if the F-c P wasn't working. Perhaps the W-D sights produced this but I'd put money on in being in the RT as well, range being the vertical scale in the tables with TE colms giving the fze setting would be my not totally underinformed guess. The GPO would interpolate the data in the tables to find the fze setting.

[Rockdoc]

Nigel, many thanks for this. You've made a bit of light dawn for me.

The Sections were certainly making allowance for wind quite early on. Meteor messages were being sent several times a day, giving wind speed and direction at one or two altitudes. The History of the Ministry of Munitions, Vol X, says that these off-sets were made and then retained for a long time as experience showed that the effect of wind variation over several hours was slight.

I'd agree that the gun layers maintained what amounted to line of sight once they had the plane in their telescopes. Like the fuze-range dial, the telescope on AA guns was geared to the elevation mechanism so that allowance for the changing effect of gravity with elevation on the shell in flight was effectively automatic. Getting the plane into the telescope in the first place was another problem.

The History describes the method of estimating range, altitude and speed using two range-takers. I'm pretty sure that this shows one of the two at 99th AA Section:

The problem with this is the need for the two men to be set at about 1,000 yards apart for an acceptable level of accuracy for the altitude, given in the Hstory as 12000 ft +/- 300 ft. That's fine for Sections like 73rd (A & B positions), 95th and 99th where two guns were stationed close to each other. Most were used as Subsections and could be a very long way apart, such as 90th which was based at Janes and along the Doiran front anywhere between Kilindir and Karasouli. There'd be no chance of two range takers per Sub so they'd have to use a rangefinder, with a considerable drop in accuracy. That ties in with a comment I came across in a Diary recently where the shots were all too frequently being reported - by patrolling Allied planes, presumably - as 'short'.

When they were close enough to see the same plane then they could go back to a wider base-line. On 14th November, 1917, 97th AAS has No 1 Gun at Gokcelli (1237-1795) and No 2 Gun at Tertre Vert (1225-1773, both Guevgueli 1/50000 map) and a line laid between them. Somebody's obviously had a bright idea and the record for that day says:

Fine again. Planes busy - over at intervals throughout day. Cross observation between guns proved very useful, saving much ammunition which would otherwise have been wasted.

I interpret that to their having to use much more estimation in their settings when they were working alone. That's bad news because the planes were rarely in sight for more than a couple of minutes, never mind in range and bearing, so there's just no time for a correction to be made. They needed to get it right from the first shell to be effective.

All grist to the mill!

Keith

[nigelfe]

Having looked a bit deeper into Routledge.

The first step was the realisation that if range and elevation angle were trigonometrically combined then it would be possible predict fuze length. They did look at a long base but too many obvious drawbacks. Instead the Height/Range Finder (HRF) was introduced. This was a Barr & Stroud UB2 coincident optical rangefinder with a 7 ft optical base mounted on a tripod (pics also show the usual field artillery 1 metre one being used for AA).

Range tables for AA were produced that enabled adjustment for non-standard conditions but I’m not clear on how this was applied to the sights, possibly it was reflected in the deflections ordered.

Next they produced the Height Fuze Indicator (HFI). This was on a small stand. A vertical quadrant plate rotated on the stand. This plate was engraved with horizontal lines representing heights and curves representing fuze settings at different elevation angles. There was a sight-bar pivoted at zero height, two operators used the instrument to track the target is elevation and azimuth. The fuze length was read from the fuze curve where the sight-bar cut the target height line. There were quadrant plates for each type of gun. Later the sight-bar gained optical magnification and heights were represented logarithmically on the quadrant. HRF and HFI became standard issue to AA sections in 1916.

At this time the deflection applied to azimuth and elevation laying was produced by the GPO (I’d guess metal arithmetic and tables, using range and perhaps a stop watch). However, the French produced the Brocq predictor, which was used with the 75mm AA gun. This used tachymetric principles, the range was set on it, it was an electrical device with variable resistors and voltages proportional to the turning rate of the instrument, this it output the azimuth and elevation deflections. It even provided direct transmission of data to the gun.

At about the same time UK produced the Wilson-Darby tracker, it too was tachymetric but was two instruments each on a stand and did not transmit data. One sight dealt with elevation the other with azimuth. Their outputs were marked by hand on a board marked with time rings in the Sect CP. The GPO used this to order deflections. The CP also had an orders board on which the succession of orders given in an engagement were written.

I've found I have pictures, instructions and description of the Wilson-Dalby Gun Director 'for the direct measurement of travel corrections', carefully marked Confidential and Patent No 6801 1915.

[Rockdoc]

<Chuckle!> I nearly understood some of that!

I've gone back to the OH Ministry of Munitions and re-read the section on range and height finding. It says that there was a Barr & Stroud instrument that was very accurate with a base length of only 6ft but that it was expensive, elaborate and did not come into general use. As it says it had a system of gear-wheels and cams, which sounds as if it would work in a factory but not reliably under combat conditions. That section also mentions a Rayner height finder and a Hartree instrument, saying the latter is similar to a French one. There are also several references to the "Bennett principle".

The OH confirms that range and height finders were used singly but stresses that their accuracy was greatly impaired and that a long base with two observing stations, linked by telephone, was always to be preferred. I don't think there's any doubt that Subsections that were isolated had no choice but to use whatever they had and hope their experience and the various tables could compensate for whatever shortcomings they were stuck with. The occasional negative comments from the War Diaries and the obvious delight of the Diary authors when a plane was hit shows that they were not always doing as good a job as they would have liked to have done. Seeing shells burst well away from the target must have been painful for experienced Artillerymen. Sections with two guns stationed relatively closely could make use of a long base and it clearly made a big difference in the case I quoted but in Salonika they were in a small minority.

Thanks for the input. I'm enjoying it.

Keith

[nigelfe]

UB 2 was the first of a line of optical HRF that lasted into WW2, the longest was 18 feet. Photos in Routledge show both the 7 ft (actually I think it was 2 m despite being British) and the shorter field arty one in use for AA. The short pattern survived field service in fd arty into the late 1960s, so I think they were more that tough enough!

Given that the number of AA sects was only in the hundreds (1 per division plus UK) I don't think the cost of UB2 was a serious issue, although B&S production capacity may have been.

An AA sect's guns seem deployed close together, ie in shouting distance of the Sect CP (which was in the open), on the W Front at least they seemed to have moved several times a day (one of the benefits of being motorised), this suggests that deploying two obsn parties some distance apart, putting them in survey sympathy and laying line was not really a practical proposition.

Routlege also gives the org of an AA sect in the BEF in 1917. Only 3 telphonists/linesmen (who presumably tapped into a local phone circuit for rearwards comms) and no surveyors, without surveyors there was no way of putting 2 obsn posns in sympathy and without sympathy there was no way of producing useful data. There were also only 4 soldiers for the UB2, and a photo in Routledge shows it seemed to need 3 to operate it. It also lists 4 men for the Wilson-Dalby trackers and 2 for HRF, and there was also the identification telescope, proably used by one of the air sentries.

When it comes to what happened in the field I'd tend to believe Brig NW Routledge "History if the Royal Artillery - Anti-Aircraft Artillery 1914-55".

[Rockdoc]

I think some of the discrepancies that are showing up are down to slight variations in day-to-day practice between the two Theatres. Effectively, there were three groupings - for the want of a word - of British AA guns in Salonika. Fourteen Sections operated in the Salonika Theatre, although three can be almost discounted as being formed in mid-1918 and saw pracically no action.

XIIth Corps had two Sections - 24th and 32nd - right from their arrival in January 1916, except for a very short time in mid-1916 when 32nd was transferred to XVIth Corps. It then gained 90th, 94th and 97th around April 1917.

XVith Corps gained 74th in October 1916 and 91st in January 1917.

In the area of Salonika, 73rd - then with two guns, later with four - became active in October 1916. 95th became active in April 1917 and 99th the following July. 98th became active in April 1917 and was on its fringes. Its No 2 Gun leaned towards XVIth Corps but was always positioned west of Lahana. 98th's No 1 Gun, not all that far away, worked more closely with the Salonika group.

XIIth Corps seemed to develop a number of AA positions its sub-Sections used as and when needed. XVIth Corps seem not to have re-occupied positions very often. In both Corps, the guns moved perhaps once a month or less, on average. The guns in the Salonika group were static.

Communications in the Salonika Theatre were very good indeed. The French established two signals stations, at Vodena and Mont St Paul, just outside Salonika itself, into which aircraft movements were fed from practically anywhere. Signals are recorded as originating from OPs, Divisions, railway stations, camps, dumps and they passed through their national signals systems to the French, who redistributed it. Interpreters were stationed at the two signals stations so that messages could be received and transmitted in any language. All AA Sections were made aware of all aircraft movements in or near the borders of the British zone. Sometimes he same message arrived by both British and French signals systems. Even the Sections near the front lines had good communications. RE Pack Wireless Sections are recoded as attached to some, though I'd expect them to have serviced nearby Sections, too. When preparations for moving position are recorded, it's usual for an officer and an NCO to visit the site and assess it first but one of the next jobs is to take "RE supplies" over and get the comms started.

Not all the Sections record every signal in detail but those that did show that they were almost certainly all getting wireless signals. Morse is sometimes shown up when AAA appears in a signal. The units reporting to Corps did so frequently, suggesting it was easy and not reliant on despatch riders, although they were part of the ASC complement of a Section. The Salonika group had telephone communications between themselves and direct links to Mont St Paul. That meant they could warn each other of aircraft approaching them, when a couple of minutes was all it took to travel the distance between them.

I fear I've moved off the topic somewhat but I think that it was possible, at least, for some of the Sections to be able to utilise long-base systems because they were in one place long enough for the required data to be gathered. That was not the norm, by any means, simply because most sub-Sections were nowhere near each other, let alone in positions to allow common sight of an approaching aircraft.

Why did Salonika Sections move so little? It may be the nature of the conflict in Macedonia coupled with the truly appalling state of the roads. Universal joints on the gun lorries were prone to failure and I get the impression that some of the positions were difficult to reach. One record I transcribed recently described leaving the GS lorries some distance away from the gun-station (as they start to call the positions in 1917), presumably meaning the ammunition had to be manhandled the last mile or two.

Great stuff, Nigell. It's making me think hard and that's always useful (not to say difficult!)

Keith

[nigelfe]

I think some of the discrepancies that are showing up are down to slight variations in day-to-day practice between the two Theatres. Effectively, there were three groupings - for the want of a word - of British AA guns in Salonika. Fourteen Sections operated in the Salonika Theatre, although three can be almost discounted as being formed in mid-1918 and saw pracically no action.

XIIth Corps had two Sections - 24th and 32nd - right from their arrival in January 1916, except for a very short time in mid-1916 when 32nd was transferred to XVIth Corps. It then gained 90th, 94th and 97th around April 1917.

XVith Corps gained 74th in October 1916 and 91st in January 1917.

In the area of Salonika, 73rd - then with two guns, later with four - became active in October 1916. 95th became active in April 1917 and 99th the following July. 98th became active in April 1917 and was on its fringes. Its No 2 Gun leaned towards XVIth Corps but was always positioned west of Lahana. 98th's No 1 Gun, not all that far away, worked more closely with the Salonika group.

XIIth Corps seemed to develop a number of AA positions its sub-Sections used as and when needed. XVIth Corps seem not to have re-occupied positions very often. In both Corps, the guns moved perhaps once a month or less, on average. The guns in the Salonika group were static.

Communications in the Salonika Theatre were very good indeed. The French established two signals stations, at Vodena and Mont St Paul, just outside Salonika itself, into which aircraft movements were fed from practically anywhere. Signals are recorded as originating from OPs, Divisions, railway stations, camps, dumps and they passed through their national signals systems to the French, who redistributed it. Interpreters were stationed at the two signals stations so that messages could be received and transmitted in any language. All AA Sections were made aware of all aircraft movements in or near the borders of the British zone. Sometimes he same message arrived by both British and French signals systems. Even the Sections near the front lines had good communications. RE Pack Wireless Sections are recoded as attached to some, though I'd expect them to have serviced nearby Sections, too. When preparations for moving position are recorded, it's usual for an officer and an NCO to visit the site and assess it first but one of the next jobs is to take "RE supplies" over and get the comms started.

Not all the Sections record every signal in detail but those that did show that they were almost certainly all getting wireless signals. Morse is sometimes shown up when AAA appears in a signal. The units reporting to Corps did so frequently, suggesting it was easy and not reliant on despatch riders, although they were part of the ASC complement of a Section. The Salonika group had telephone communications between themselves and direct links to Mont St Paul. That meant they could warn each other of aircraft approaching them, when a couple of minutes was all it took to travel the distance between them.

I fear I've moved off the topic somewhat but I think that it was possible, at least, for some of the Sections to be able to utilise long-base systems because they were in one place long enough for the required data to be gathered. That was not the norm, by any means, simply because most sub-Sections were nowhere near each other, let alone in positions to allow common sight of an approaching aircraft.

Why did Salonika Sections move so little? It may be the nature of the conflict in Macedonia coupled with the truly appalling state of the roads. Universal joints on the gun lorries were prone to failure and I get the impression that some of the positions were difficult to reach. One record I transcribed recently described leaving the GS lorries some distance away from the gun-station (as they start to call the positions in 1917), presumably meaning the ammunition had to be manhandled the last mile or two.

Great stuff, Nigell. It's making me think hard and that's always useful (not to say difficult!)

Keith

Being static would make a long base possible, and towed guns would reduce the inclination to move frequently. I guess the War Office had Salonika at the back of the line when it came to modern equipment like UB2, however, there were the small RFs used. Basically it is so much simpler to use a RF, measure the range and elevation angle and use a simple table or card to find the height (if the instruments doen't do it automatically). Survey would have been done by the surveyors of the RE Army Survey Battalion, who surveyed all artillery positions.

[Rockdoc]

Being static would make a long base possible, and towed guns would reduce the inclination to move frequently. I guess the War Office had Salonika at the back of the line when it came to modern equipment like UB2, however, there were the small RFs used.

Salonika always was well down the list of priorities - about 99th on a list of one to ten. Few in the British corridors of power wanted anything to do with the place but there was always the need to be seen to support the French, who were much keener, although they also had plenty of top brass who saw the Western Front as the only Theatre, too. It's undoubedly true that support ebbed and flowed in both London and Paris.

Basically it is so much simpler to use a RF, measure the range and elevation angle and use a simple table or card to find the height (if the instruments doen't do it automatically). Survey would have been done by the surveyors of the RE Army Survey Battalion, who surveyed all artillery positions.

I'm sure they did whatever they could with whatever they had available, as any Artillery unit would do. I'd guess there was some frustration that they didn't bring down or even divert many planes, which may have been partially down to the relative inaccuracy of using a range-finder in some cases, but Centurion's also poined out that the vast majority of missions in this Theatre were for reconnaissance, not attack, so the planes rarely flew below 10,000 feet and were getting higher as time went on. With 13-pdr 6-cwt guns, the 1600 ft/sec muzzle velocity meant very long flight times to those altitudes, even with a long-base finding system in use. The superior 13-pdr 9-cwt guns were also going to struggle as there are so many manoeuvres that can be made in 22 seconds, even if the initial settings proved to be spot on.

Keith

[Rockdoc]

Nigel,

Kate & Martin Wills sent me this photo today and it shows No 1 Gun of 98th AAS at Guvesne, while operating as a Subsection. In the foreground is a telescope. Is this one of the devices we've been talking about?

Keith

[Rockdoc]

I've just begun to transcribe 73rd AAS' Diary for January 1918 and it reports the first use of a height finder. They seem to have been quite enthusiastic about its potential, although it clearly hadn't been an instant success. I'm guessing that this might well have been at their "C" Position simply because that was working as a Subsection, while the "A" and "B" guns were at least within a reasonable distance of each other so could have used a long-base system.

Keith

[Stumpy600]

The references to the HFI in this thread are of particular interest to me.  I posted yesterday in 'Other Equipment' about an HFI  made by Newman & Guardia (the camera makers). It's dated 1925 and appears to be incomplete.  It bears no resemblance to the WW1 HFI's I've seen photographs of.  As its only seven years after WWI I guess maybe early examples of this HFI could have been in service then?  The other great confusion is the bearing scale.  If the instrument is for height/fuze length calculation the bearing is an irrelevance surely?  Any thoughts on this would be greatly appreciated!