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Crich Police Box Lamp - 2014 Dimensions

Started by matt sanders, Aug 25, 2014, 06:39 pm

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matt sanders

I recently visited the Crich Tramway Museum with Mark, and we got lots of info about their Police Box, though this thread is mainly about the Lamp.

As well as taking dimensions, Mark took lots of photos, and I made a silicone mould of a quarter of the Lens, so I have now extrapolated an exact profile for the Fresnel ribs.

Meanwhile, Starcross recently surveyed the Police Box Lamp that lives at Banstead Police Station, (see that thread), which has helped fill in a few gaps in the info that we had, as well as highlighting certain variations between the two lamps.

I'll therefore post some diagrams of the Crich Lamp, starting with a Side Elevation with dimensions - note the colour-coding.

NB:  These are work-in-progress diagrams, and certain issues require more information/dimensions, and some discussion of theories about the assembly.  So, please don't distribute these WIP diagrams elsewhere - we'll have more definitive versions soon...


matt sanders

A few things to note:

Slanting Surfaces:
-  It seems that all "horizontals" are not horizontal, but actually slant, to throw off the rainwater.
-  Even the "verticals" are not vertical, perhaps to aid release of the metal castings from their moulds.

Slanting Rods:
-  Note how the four Rods are also slanting - by accident or design...?
-  The Spare Base has holes where the rods previously attached - but they are spaced further apart than those on the Lamp-Base on top of the Crich Box.  If this Dome were placed on the Spare Base, the rods would probably be vertical.
-  Perhaps, during the Hendon refurb, lamp parts from different boxes got mixed and matched...?
-  Or maybe the slanting rods are a design feature - has anyone seen them elsewhere...?

More to follow shortly...

matt sanders

Here are a couple more views:

Top View, and a Horizontal Section, through the base of the Lens, looking down on the square Base.

When I was at Crich, I didn't actually notice the four screw-heads on the top of the Dome, but they are clearly there on Starcross's Banstead images. 

Maybe Crich is clogged up with paint - does anyone know of any photos that clearly show the top of Crich's Dome...?


matt sanders

And here are two Vertical Sections...


-   The screw-heads at the top are visible in Starcross's Banstead images - see the Banstead thread.
-   The Nuts at the bottom are visible on the underside of the Spare Base - see Mark's photos, which will follow shortly.
-   Therefore, I have suggested a construction arrangement on the Diagonal Section - but does anybody have a better theory...?

Base Interior:
-   We didn't have time to take full dimensions of the Spare Base's interior/underside.
-   Instead, I have scaled from Mark's photos, but I'm pretty sure it's all correct, as the photos were from helpful angles, and they suggest nice round numbers in imperial measurements.

-   I have assumed a metal thickness of 3/8" on the top Dome - and my mould and some photos back up that assumption.
-   Note that on the Banstead Lamp, the dome has a slightly different profile, with wider "flat" ring.

-   I can confirm that this is exactly the shape of the Crich lens, down to the fact that the top edge of the bottom rib has a slightly bigger radius than the others.
-   As the bottom of the glass was obscured by the metal base, I have had to assume that it sits directly on the metal ring below.
-   I have also assumed a glass thickness at top and bottom of ΒΌ", but this would be easy to confirm on site, with a bit of bent wire poked in the gap at the top.
-   The Banstead Lens has more prominent ribs, and taller straight sections at top and bottom - I'll try and post a direct comparison shortly.

-   Note that I have indicated some Putty, securing the Lens to the Base, and sealing the join.
-   I don't know if this is actually what was done, but it would make sense, to stop water getting in.
-   It would also explain/justify the width of the metal ring that the lens sits on.
-   The Crich Lens is securely fixed in place, but the bottom ring of the Fresnel has been painted blue, so I don't know if we can confirm this putty detail on the inside. 
-   At Banstead, the lens moves freely, but of course that wasn't an "official"  police-box installation, but a bodge-job re-using the lamp.

Fixing to the Roof:
-   Does anybody have theories about how the metal Base was attached to the concrete Roof?
-   I wondered if it might just be cemented on, but note that in each of the four sides is a Weep-Hole, to allow water to escape, so cement might have fouled that drainage.
-   Perhaps there was an additional metal bracket, that isn't shown here.  But note that the top of the concrete shell has five holes in it, which are not blocked.  (Central hole for lamp cable, and four ventilation holes.)

Anyway - does anybody have any comments or suggestions, or know of factors that I have overlooked...?

matt sanders

Here are some photos of the Lamp, from the day that I made the silicone mould:


matt sanders

matt sanders

And Mark's photos of the Spare Base...


matt sanders

Does anybody have good info about the type of light-fitting that would have been in the lamp?

And any ideas what the domed thing is, above/around the bulb, in this diagram...?


On this diagram it is labelled, but I can't read the writing.  "Coloured Cylinder"...?



Aug 25, 2014, 09:52 pm #11 Last Edit: Aug 26, 2014, 05:44 am by hb88banzai
Best guess I've been able to come up with on the lamp is something like one of these --


An Edison vacuum bulb. Different gas mix (or no gas at all) than the usual incandescent. Don't think they were too particular later on, remembering these GPO plans are from the fall of 1937.

As to the dome, that is a glass dome that was tinted red (labeled a "coloured cylinder" in the Trench plans). These remained standard until, if I recall correctly, early 1937 (I'll have to dig up the exact date) when all the Boxes were changed over to clear/white blinking signal lights at the same time. So yes, absolute confirmation, Met Police Box lights were originally red like the Dinky toy, but the Fresnel itself was always clear. There was quite a long and convoluted story to all that, but the changeover had been in the works for a very long time and was tied in to the whole telephone system saga, among other things.

This means the dome depicted was already obsolete when the GPO plans were drawn up, but there you go.

Would have been like this (but red) from an anniversary clock --


Or perhaps like this (a flange at the base would be more stable and durable, just a question of it would fit) --


... though probably more cherry red than orange red (unlike in the provinces).


Quote from: matt sanders on Aug 25, 2014, 08:42 pm
Does anybody have good info about the type of light-fitting that would have been in the lamp?

The light socket Fitting is called B22 for Bayonet fittings. They still get used in the UK so they are easy to find at a B&Q if you want one. I'd have to search for my dimensioned line drawing however.


Its my theory that the conduit extended up through the concrete and the lantern base went over the hole in the middle of the bracket capped by the light socket. Thus keeping the lantern base in place. You can see it in the GPO drawings now that we have photos of the spare base to compare it to.

As an aside, the Police Posts, and the PA1 lanterns uses this sort of fitting.


Quote from: hb88banzai on Aug 25, 2014, 09:52 pm
As to the dome, that is a glass dome that was tinted red (labeled a "coloured cylinder" in the Trench plans)....the Fresnel itself was always clear.

It might be important to note that the PA1 lantern glass I have has a section of red reflective coating inside the glass lens. This effectively colored the glass red, but over time it was washed away by the elements, and it was left a basically clear glass. Later replacement glasses came in the tinted glass in a sort of rootbeer amber.

I'll see about getting a close up photo of the material.


Aug 26, 2014, 05:56 am #14 Last Edit: Aug 26, 2014, 07:04 am by hb88banzai
Quote from: starcross on Aug 25, 2014, 10:19 pm
The light socket Fitting is called B22 for Bayonet fittings. They still get used in the UK so they are easy to find at a B&Q if you want one. I'd have to search for my dimensioned line drawing however.


That would make sense, as bayonet types are a lot more secure over time, not being subject to self-loosening from thermal expansion.

Note that the Edison type vacuum lamp shown above does indeed also come with B22 type bases, and of course in the specified 60 watts.

Here's one (though they're not cheap, being marketed as vintage/retro) --


I don't know for sure that this is the type of vacuum bulb they used, but the shape shown in the GPO plans is pretty suggestive that these are the type of bulbs being specified. One reason they may have been used instead of the more common (by then) short-filament gas-filled incandescents (as used inside the Met Boxes) is that the Edisons were somewhat notorious for being very long lived (though they did tend to dim with age), and the top light is not something you would want to have to replace very often, especially on an emergency basis.