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 that 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 Watt 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 Watt 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 late 1960s.

The lantern is of a functional design with little in the way of embellishment. 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!


Owing 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 it 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 The original terminal block (visible on the left) has been replaced by a smaller, more modern one at some point.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. A circular reflector plate would have been situated above the lamp when the lantern was new; this has been lost through the annals of time.

The thick glass 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.

Restoration of the lantern began on Sunday, 3rd June 2007. I discovered that the top part of the lantern's body was actually removable - this helped immensely as the lampholder assembly was now more accessible. I managed to free the assembly using a lot of 'persuasion' - only the thicker of the two focusing rods managed to defeat me! The top of the assembly can be seen bottom-right. I discovered that several other screws were fitted to the internal rim of the lantern's body - they were positioned on the underside at 120 degree intervals which I can only assume would be to hold a reflector in place. If one was ever fitted in this lantern, it is long gone now. The bowl securing ring (visible top-right) simply unclips from the frame.

With the main body of the lantern now in pieces, I now concentrated on the porcelain GEC lampholder. This was filthy, and the copper of the thread was heavily corroded. There turned out to be more components in the lampholder than I had expected!

The porcelain appears relatively clean here as most of the dirt had transferred to my hands at this point!

A lot of dirt and grease had managed to become trapped around the text on the underside of the lampholder. The majority of this was removed by soaking the porcelain in bleach and then water containing washing powder...

One (almost) squeaky-clean piece of porcelain!

Meanwhile, the metallic components had been dipped in vinegar to remove as much corrosion as possible. Although not perfect, the end result was better than I had expected. The components were then re-fitted to the porcelain body...all ready for a lamp - whatever that may be..!

The lampholder assembly was also cleaned up - I was surprised to discover that it was made of brass.

The lantern's frame returned from having new grub screws fitted on Wednesday, 20th June 2007. Apparently, the old grubs put up a real fight due to them being made of hardened steel. The new screws are stainless steel and are metric rather than imperial. The jammed support rod was also freed.

What was left of the original bracket was retained; albeit, with with two fresh drill holes from when the grub screws were removed.

The replacement bracket consisted of a special side-entry AC Ford AC184 with a 42 mm sleeve had already been ordered and installed - all ready for the lantern!

Once the lantern had returned from receiving new grub screws, I decided to fit it temporarily to the bracket to see how it looked. As I didn't have the lantern frame available when I was fitting the bracket, I had to guess at what height to mount it at. Bizarrely, the height I eventually went for was just right. The bowl had been thoroughly cleaned out by this time, although it will be cleaned again when the lantern is ready to be powered up. Surrounding the lantern are other GEC models - the Z8896 on the left; the Z8260 just below and the Z9481 on the right.


The terminal block had also been released from the frame - when turned over, yet another GEC logo stared back at me!

The canopy, frame and bowl ring were bead blasted on the weekend of Saturday 23rd-Sunday 24th June 2007. When they returned on the 25th, I fitted the lampholder support poles and then the lampholder assembly itself.

The components were then fitted together, although this was, again, only temporary, as the metalwork now required painting.

The screws for holding a reflector in place were now much easier to see:


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 Watt 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 tube. 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 re-fitted the lantern's top.

The underside of the bowl features arrows that point in the direction of its two main beams.

This view is taken from the side of the lantern - looking directly at the refractors that make up one of the aforementioned 'main beams'.

One final 'daytime' image to show the lantern in its fully-reassembled form.

Lantern warm-up video:

Testing with my energy monitoring device revealed the following results. Naturally, with the capacitor disconnected, the Power Factor was less than impressive.

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
234.3 1.19 261 279 49.8 0.93 259.30 9.30 4%

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 that they had been installed only recently when this photograph was taken.

The photograph originates from the excellent 'Picture The Past' ( website and is credited to Derby City Council.

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