Francis Barker Mk III Prismatic Compass

There are many tragedies that can befall a mechanical watch: dirt or grime can gum up the works, a slight fall will break pivots and jeweled bearings, moisture will oxidize steel pinions in a heartbeat, and magnetism can enchant a hairspring ruining the balance wheel’s consistent beat. When examining a malfunctioning movement under the loupe, grime and oxidation are easy to discern. Broken pivots and jewels will also come to light during early testing. A magnetized hairspring can sometimes be difficult to discern.

Fortunately there is a simple solution to this problem. Savvy watchmakers have discovered that simply placing a compass next to a watch afflicted by magnetism will result in the compass pointing at the watch instead of magnetic north. A handheld compass is therefore an important part of any watchmaker’s tool kit.

I used to own a nice lensatic compass like those issued to soldiers of the US Armed Forces.  The compass had affixed a small lens to make reading easier.  It was liquid dampened and also fit nicely in the pocket; unfortunately it is now lost to time. As a person who appreciates finely crafted instruments, I set about finding a suitable replacement which would excel in both form and function. I settled upon a vintage Francis Barker & Sons Mk III Prismatic Compass.

The Mk III was produced by the famed compass manufacturer Francis Barker of London during the first half of the 20th Century and was used primarily by the British forces during the Second World War. It is a well designed precision instrument that allows soldiers to orient to within a half a degree of accuracy. Also, since the compass was treated liberally with luminous paint, it is just as functional in the dead of night.

I purchased my Mk III compass from a dealer in Greece who kindly shipped it stateside. The compass has had a harsh life though and would need restoration before returning to service.

The Mk III is large, about sixty millimeters in diameter, and made of heavy brass. Inspecting it, I discovered all the glasses were still intact and the hinged prism functioned as expected. The bowl housing the compass card had long ago lost the dampening fluid which resulted in the card swinging wildly to and fro before settling; however, this suggested the jeweled bearing was still intact. To my relief the luminous radium paint had been removed by a previous owner.

Stamped on the underside of the compass were the letters CKC which confirmed that this particular compass was manufactured by the Canadian Kodak Company under license from Francis Barker & Sons. The Broad Arrow also confirmed the compass’s military heritage. The year of manufacture is 1943.

The rubber anti-skid pad on the bottom of the compass had hardened and begun to flake off, thus during restoration it was the first bit to be done away with. I then turned the compass over and removed the rotating bezel with inset crystal.

A quick test with my dosimeter confirmed residual radioactivity from the luminous paint so extra caution was utilized during disassembly and cleaning.

Eight screws aligned radially secure the lid to the bowl. With these removed I extracted the crystal and compass card.

At some point water had entered the compass bowl and oxidized the iron needle beneath the card. Rust was found all throughout the interior of the bowl and had also badly stained the crystal.

The compass card is cut from mother of pearl. This material is translucent allowing the light from the luminous paint applied beneath the card to pass through and illuminate the compass rose. In 1943 this was a rather innovative solution which allowed a soldier to orient himself even when the sun was below the horizon.

Sadly the luminous compound contained copious amounts of radium which is highly toxic. In addition to receiving an unwelcome dose of gamma radiation to the face when orienting, soldiers put themselves at risk of alpha particle ingestion if they didn’t wash their hands before eating.

Although most of the radium had been removed, oxidation had bonded radium particles to the needle resulting in a fairly hot compass card.

The bowl was removed from the compass housing which gave me my first look at the filler plug. A small fibre washer sits between the filler plug and the bowl but for good measure the plug was also sealed externally.

Deconstructed, each part was carefully cleaned. The housing was stripped of paint as well before receiving a brand new coat.

The external lettering was repainted under the microscope using some flat white paint and a thin oiler. A new black rubber anti-skid pad was applied to the base of the housing.

The bowl lid and screws were replated with silver and the compass card carefully cleaned with spirits which returned luster to the mother of pearl.

One of the screw holes in the bowl was stripped out so I re-tapped it and a replacement screw was turned on the lathe to match the dimensions of the newly tapped hole.

I plated the new screw in silver to match the others.

Where possible tritium tubes were installed as a replacement for the original radium based paint. The tritium will glow non-stop for up to ten years and does not require recharging under light. Where the tritium tubes would not fit or would appear ungainly, I utilized a Europium based luminous compound which glows quite bright after a short charge in natural light.

The crystals were cleaned and black enamel applied to any markers which needed it. This was followed by a thorough polishing.

I reassembled the compass bowl placing the compass card on the pivot then filling the bowl with purified kerosene. The crystal was replaced followed by a new flourosilicone o-ring. I chose flourosilicone as it does not degrade when in contact with kerosene. The lid was secured with the eight retaining screws. It is important to torque these screws in a star arrangement to ensure the lid is pressed firmly and evenly on the bowl.

Topping off the dampening fluid was done with a small syringe inserted in the filler hole.

I stripped off the worn lacquer from the coin edge bezel and polished it before applying a new coat of clear lacquer. The lacquer will ensure the brass bezel remains bright with repeated use.

A new crystal was required for the hinged lid as the existing one was damaged (chipped). A replacement was ordered from my parts supplier and etched to add a sighting line. Etching was accomplished with a bit of etching cream and some packing tape applied as a mask.

The newly etched line was then filled with black enamel, the excess being polished away after curing.

The compass was then fully reassembled.

Assembled and filled to the brim with dampening fluid, the compass is a substantial instrument. I haven’t weighed it yet but I would guess it’s about a half a pound of weight in the hand.

Although it will be used primarily to test watches for magnetism, I did want to make sure the compass performed in its original duties as well. Tested against the compass application on my mobile phone I registered a deviation of one degree (I’d add a picture of the two side by side but putting the phone near the compass results in further deviation).

I also took the compass out in the dark to see how well the tritium tubes work as a replacement for the luminous (radium) paint and I must say I am very impressed by the engineering of this instrument. The tritium illuminates the compass rose bright enough to easily get your bearings during the night. The use of a prism mounted over the card is ingenious. The user can see the illuminated card reading magnified in the prism while at the same time sighting a landmark; furthermore, the dampening fluid removes any jitter normally associated with a compass- it turns as smooth as silk.