GEC Clearmain 'Difractor' Z8128B

Lantern acquired in May 2007.

This classic mercury lantern was located in the backyard of the old Co-Op buildings on Handel Street - located near Osmaston Rd in Derby City Centre. Jeremy knew it since 1986 - the year he started with a firm who occupied the premises at the time. He reckons that at one time it would have been owned by the Council (or Corporation back then) as it had a live EMEB (East Midlands Electricity Board) service and its own time switch. The mains came overhead from the nearby overhead house services. It was mounted on a wooden pole carrying the overheard wires. The lantern was maintained until 1999, using whatever 250 W MBFTs were on the shelf at the time. It may originally have had an MAT/V lamp fitted as control gear was never installed.

Now retired into the collection, the lantern has been fully restored and, thanks to the addition of gear, runs a 250 W MA/V lamp. 

Unsurprisingly, examples of this enormous lantern cannot be found on public roads in Derbyshire these days; though photographs taken a couple of years after the Second World War show examples installed in the City Centre. It is difficult to say when these were replaced but I would think it would have been in the 1960s.

The lantern is of a functional design with little in the way of embellishments. The 'Difractor' name relates to this type of bowl - different bowl types had different names.

The GEC logo is stamped on the lantern's spigot. A length of the old bracket remains in the spigot - that should be fun to remove!

Due to the thick glass bowl, the lantern becomes top-heavy when positioned upside down and so a handy Thorn ballast was drafted in to steady the lantern whilst I took the photograph! In the centre of the bowl are the words 'Main Beams', along with arrows pointing in either direction.

The insides of the lantern are blackened and severely corroded from years of heating. Even the asbestos wiring to the lampholder has seen better days. The original terminal block (visible on the left) has been replaced by a smaller, more modern one at some point. The PVC wiring to this is burnt to a crisp and of course will be renewed when the lantern is restored.

The bowl is probably the item in the best condition out of everything - it is in desperate need of a clean but appears to be undamaged. An etch on the top of the bowl (as it appears here) dates it to 1952.


Click here to see the lantern's restoration



Following the lantern's cleanup, the metalwork was then repainted in a hammered grey finish. The lampholder was then refitted and wired up - I decided to wire directly into it as I couldn't find a modern porcelain terminal block which fitted exactly as the old one had done. Modern heat-resistant cabling (complete with fibreglass sleeving) was used in place of the old asbestos cable. As for what lamp I used in the end, the brass cap should be a clue..!

Only an OSRAM 250 W MA/V! The lantern was now all ready to be switched on, however I was concerned about doing so after hearing stories of MAT/V lamps rupturing due to aged arc tube seals. Still, it would be a shame not to try at least - so after a deep breath, I flicked the switch...

...and on it came, along with some accompanying spitting from the solid mercury blobs inside the arc tubes. As Phil Macbean mentions on his 'MA/V lamp warming up' page, the arc fills the arc tube to start off. The lamp made an interesting pattern on the bowl.

The arc began to thin within a few minutes of being switched on.

 

With the lamp now at full power (notice the red-hot glow on the top electrode), I fitted the lantern's top to try to gain some protection against the powerful UV rays being produced.

The underside of the bowl looked like a scene from The Matrix!

This view is taken from the side of the lantern - looking directly at the refractors which make up the 'main beam'.

Lantern warm-up video:

This photograph taken on the 27th September 1947 shows Clearmains in use on 'The Spot' in Derby City Centre. A photograph taken the previous year does not show them - suggesting to me that they had only recently been installed when this photograph was taken.

The photograph originates from the excellent 'Picture The Past' (www.picturethepast.org.uk) website and is credited to Derby City Council.

Urbis ZX1 | GEC Z9536


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