Thorn Gamma 3

Lantern acquired in February 2021.

This lantern was one of two that were situated in the Market Place area in the Derbyshire village of Crich. This one was removed when its column failed a structural test that the local Parish Council instigated, requiring a new column to be installed. Owing to parked cars being in the way, two previous attempts to replace the column were thwarted, but it was very much "third time lucky", and the column was removed successfully in mid-February 2021, with the lantern (and its control gear) being saved into the Collection; thank you to the contractors for doing this - I hope that the chocolate Digestives were acceptable! Several examples of the Gamma 3 existed on Derby's East Street for many years until refurbishment of the street in the late 2000s saw them removed. Two further examples, fitted to 5 m columns, existed in the 'Duckworth Square' shopping precinct in the City Centre, until this was demolished earlier in the same decade. Thus, the one surviving example in Crich is now the last known Gamma 3 on a public road in Derbyshire. Outside of this area, another notable Gamma 3 installation was at BBC Television Centre in London, where multiple examples were installed on the roadway at the front of the building (being fitted to the same columns that supported Gamma One fluorescent lanterns when the complex opened in 1960) prior to the site's redevelopment in the mid-2010s.

The lantern is seen here in August 2011. An archival photograph taken from a similar angle shows that a 15 ft cast iron column supporting a two-way ESLA lantern was installed in roughly the same position in the past. The classic K6 red telephone box also features in the photograph; as does the nearby signpost (seen here prohibiting parking between the hours of "1 A.M. and 6 A.M."), although the pole was painted black and white back then, and also supported a Parking sign.

Originally, the two lanterns were switched using Royce Thompson P9 photocells mounted between the lantern and the column; however, more recently, conventional two-part photocells were installed instead, with the detector mounted on the canopies, and the relays installed in the column bases.

The Gamma 3 is the 'medium' lantern out of the three lanterns that included the large Gamma 4 and small Gamma 5, and measures 636 mm (25∑25 inches) in width and 445 mm (17∑75 inches) in height. A small section of the column remained attached below the P9 casing - the contractors didn't like to chance the many rather rusted grub screws during the lantern's removal!

The P9 cell appeared to be suffering with water ingress, judging by the condensation that had formed on the lens.

"Royce Thompson Electric Ltd." is cast to the side of the lens aperture.

Pulling the cell away from the casting revealed that its wires had been cut. Sadly, the tape label that may have listed its age had become blank over time, although I believe that it could be a 1970s' example.

This close-up reveals that the writing moulded into the rubber front is still perfectly legible, however.

Remarkably, the column stump released with ease - one of the three grub screws was missing from the P9 casing completely, and a second unscrewed without requiring any additional effort. I had wondered how the P9 casing gripped the column, given the difference between the two diameters, but this was revealed - a 76 mm (3 inch) diameter spigot slotted onto the column's own tapered spigot, allowing the difference to be accommodated.

The internal construction of the spigot adaptor is shown here, with the inner part ending with this four-leaf clover-shaped piece that the column's own grub screws lock into place.

A section of the (rather buckled) canopy has been polished to a shiny finish by the overhead telephone wires that rubbed against the aluminium over the years. Three stainless steel toggles secure the canopy to the rest of the lantern - earlier versions of the Gamma 5 employed a similar system (albeit, with two toggles instead), but later versions saw this setup changed to two slotted screws. The Gamma 3, however, was never altered. The Royce Thompson ER4N two-part photocell detector is offset on the canopy.

The lantern's translucent bowl provides a diffused 360 degree output for the lamp.

A rather sooty mark exists in the centre of the inside of the canopy - this being the point directly above the hot lamp. Also on the inside of the canopy are three guides that help to locate the canopy onto the bowl.

The label states that the lantern is suitable for a single 250 / 400 Watt Mercury Vapour (MBF) lamp, although a 150 Watt High Pressure Sodium (SON-T) lamp was installed upon acquisition. The lantern is made to BS 1788: 1964 (Street Lighting Lanterns for use with Electric Lamps) - this being an amendment to the original BS 1788: 1951 (Street Lighting Lanterns - Electricity and Gas), but was, itself, superseded by BS 4533-103.1 (1981) (Luminaires - Performance Requirements - Specification for Light Distribution from Road-Lighting Lanterns); again, suggesting that this lantern was produced in the 1970s.

The cable supplying the detector emerges from a small gap in the centre of the lantern, alongside the lampholder.

Apart from a small swarm of dead wasps, some fragments of glass from a previous lamp remain in the centre of the lantern. Typically, the plate that secures the bowl to the centre casting is secured with three screws, all of which are resolutely stuck! The current SON-T lamp was made in August 2013 (3H). The lampholder looks to have had a label affixed to it originally - again, any information that it contained has long since disappeared.

I suspect that the two lanterns were installed with 250 Watt MBF lamps when new, but were converted to 150 Watt SON later - the capacitor carries the handwritten date of September 1988, which may be when the conversion occurred. The ballast and ignitor are both later replacements, while the ER4N relay was fitted in 2010.


Major engineering work was required, in order to separate the various components that made up the lantern. This was completed expertly by TAS Engineering of Burton-upon-Trent. The most laborious component was the cast iron base casting, with its seized, hardened grub screws securing it to the P9 casting proving particularly problematic. Gareth, the owner of TAS, explains below how he managed to free them:

"To remove the grub screws, they were heated and allowed to cool with release oil being sprayed on that then draws in as the part cools. For the very stubborn grub screw, I ground off the excess inside the fitting and then rested it on a large stainless steel screw that pressed into the socket (which had a broken driver bit in it) and then melted it with my TIG welding torch. This fused the new screw to the remnants of the grub screw and the heat loosened the corrosion. I was then able to put a pair of Mole grips on the sacrificial screw and extract it without damage to the base of the fitting. Quite time consuming but satisfying to know that the thread didnít need to be tapped oversize."

The two turrets that raised the lampholder were also in especially poor condition, and had to be rebuilt. Once this work was completed, the casting was taken to R.L. Dumelow & Son, also of Burton, for sand blasting and repainting in gloss black. New 1/2" long, 3/8"-16 UNC grub screws would be fitted later.

With the centre cleared of wasps, the part code 1046 1050 became visible.

Amazingly, the P9 casting turned out to be cast iron too (later versions were cast aluminium instead). It was also sand blasted, and received a coat of hammered grey paint.

With the lantern's base casting no longer being attached, the 'Ref A1' text is visible; this refers to the type of housing that the P9 is placed in - a separate type (the 'A2') saw the photocell installed between the bracket and lantern on top-entry installations.

TAS filled in the small hole in the canopy that the later two-part photocell had used, and also tried to re-shape it to something akin to its original circular design. The exterior was then painted light silver.

The interior, meanwhile, was painted gloss white. The clips (and the remains of their fixing bolts) were removed prior to painting.

TAS also fabricated a miniature column for the Gamma 3 to be installed on, which was then painted dark green (RAL 6009 - Fir Green) by Dumelows. I applied the lantern's former column maintenance number to the new column using stencil numbers and white spray paint. With the column assembled, the P9 casting was attached. Three new 3/8" long, 3/8"-16 UNC grub screws were fitted to the casting.

The final items to be fabricated were replacement steadies for improving the rigidity between the bowl and the canopy - the old and new versions are seen below. A piece of foam gasket strip was applied to the new pieces as replacements for the rubber gaskets applied to the old pieces.

New M5 ◊ 16 mm slotted pan head screws were used in securing the canopy retaining clips.

On the inside, locking nuts were employed to hold the whole assembly together.

Here is the view of all three steadies installed.

The bowl (and its sealing gasket) were cleaned.

I hadn't appreciated that the holes drilled through the bowl support ring, the bowl itself, the rubber sealing gasket and the lantern's base casting would have been positioned by hand during machining - this only transpired when none of the fixing holes lined up as I attempted to re-attach the bowl, after having wired up and fitted the cleaned lampholder. Only when all of the drill holes for all of the variables were in their original positions did they then line up - rather infuriating!

Eventually, the lantern was fully reassembled on its new post. The P9 is not connected (nor had I intended for it to be).

The lantern is pictured alongside the AEL Comtempo 245 as a comparison, with the Chris Cyprus picture situated between the two.

A new, old-stock, 250 Watt Philips 'PowerWhite' MBF lamp dating from February 1970 was installed, in order to return the lantern to its expected 'original' condition.

The high brightness and 50 Hz flicker picked up by the camera played havoc in trying to photograph the lantern working without appearing blurred or under-exposed. After a few attempts, I managed to capture the below image looking up at the lantern.

The lamp control gear is housed in a galvanised adaptable box located at the base of the miniature column. The box was adapted further by having two lengths of Signfix rail riveted to the back, and two clips then securing the box to the column shaft.

Lamp warm-up video:

Testing with my energy monitoring device revealed the following results:

Test Voltage (V) Current being drawn at full power (A) Measured wattage (W) Apparent Power (VA) Frequency (Hz) Power Factor True Power (W) Difference to rated wattage Percentage Difference
241.3 1.33 271 321 49.9 0.82 263.16 13.16 5.26%

Thorn Pilote T1 | Thorn Beta Six / AEI Saxby


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