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. At the time of writing, the other example continues in service, but 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. 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 (2525 inches) in width and 445 mm (1775 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.

Thorn Pilote T1 | Thorn Beta Six / AEI Saxby


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