This is the first in a short series of posts on a particular work of art masquerading as an obsolete military direction-finding instrument. My intent with this series is to provide the reader with a wealth of information on how to identify these valuable artefacts, how you might repair or restore them if necessary, and most importantly, how to use them for the purpose for which they were designed so they may enjoy a second or even a third useful working life.
The Mk. III Marching Compass is a WWII-era British and Commonwealth military compass known for its extreme accuracy in the hands of a competent navigator. Besides its military career the MKIII has, since its appearance in 1932, been an instrument of choice for surveyors, explorers, archaeologists, Scouts and even Australian stockmen and farmers.
It’s almost completely brass, with a mother of pearl dial card (see diagram). It works in a similar fashion to a USGI lensatic compass that many are familiar with, except instead of having a lens, it’s got a prism which allows you to read the bearing/shoot the azimuth while looking at the target (see other diagram). The Prismatic is a work of art, but for normal use it’s not really all that much better than a lensatic – despite being on average around 5 times the price. However, for those who needed a super-high degree of accuracy such as surveyors and soldiers calling in artillery strikes, the 1/4 of a degree resolution of the British pattern prismatic compasses made it several magnitudes better than the US lensatic compass with its 5 degree resolution. The prismatic also had one serious shortcoming when compared to the more modern Silva baseplate-style compasses – to work directly with a map most effectively, you’ll need a protractor.
This particular MKIII compass is a genuine WWII model (Mk III), which at some stage in its more recent history received a complete overhaul. The manufacturer of my particular compass is known for being extremely generous with the radium while luming the various parts of the compass during manufacture. What they did was mix glowing radium powder with an aluminium oxide paste to make a thick paint which was applied to the north markers, the lubber line, a marker under the card, and a couple of smaller markers on the inside of the lid in line with the hairline. Even after it lost its glow, this stuff was extremely radioactive at a rate of toomanymiliseiverts per hour. Dials were burned brown by the radium and they can remain a seriously hazardous item to this day.
At some stage in my compass’ life, and more than 10 years ago, it was completely refurbished, probably by Trademark. The nasty radium was cleaned out and replaced by tiny tritium illumination capsules (also radioactive, but less likely to give you bone cancer). In my compass, the tritium capsules no longer glow, and since they only last 10-12 years, I can surmise that the restoration/refurbish was done more than 10 years ago.
- The prism housing has been replaced with one off the later M73 prismatic compass, and the rubber pad on the base is black, not red, meaning it’s a replacement too. It also had the hairline window guard added. Despite the upgrades, it’s still in Degrees, so these weren’t half-arsed army Quartermaster upgrades, which would have seen a Mils dial added.
This afternoon I compared the compass’ accuracy to my Silva, my Cammenga lensatic compass and my Galaxy S4 phone. It’s still dead on – maybe half a degree out. I had to adjust the hairline (see diagram) window a little since it was off kilter, but that’s it. I’m chuffed to have found a useable 70 year old prismatic compass.
It’s a cool bit of gear, but it’s very different to the Silva orienteering-style compasses most outdoorsy folk are familiar with. Some might even argue that in the age of GPS-enabled mobile phones and very powerful outdoor navigation apps, the days of the compass are numbered. I tend to disagree, but to each their own.
It’s all fine and dandy to have an antique, but fully operational compass, but there are two issues anyone who finds themselves inspired by this series needs to consider, particularly when we get to the post about stripping one of these babies down and having your way with it, these are;
1. 90% of the genuine military prismatic compasses dating from prior to the 1970s are HAZARDOUS. They were packed with radioactive radium and the “dirty fluid” inside the compass is radioactive waste. It’s something you’d be better off letting the professionals handle for you. One such professional outfit is Trademark London – http://www.trademarklondon.com/
2. 90% of all the prismatic compasses you’ll find for sale are fake. I’m not kidding, Indian workshops are punching these out by the palletload. To the casual observer they look like MkIII Marching compasses, may have all the right stamps, may even be assembled using some genuine components, but they don’t work. They are sold for the purpose of looking pretty while sitting on someone’s desk or in a shadow box with Grandpa’s WWII medals and DSC citation, which is fine, but some unscrupulous sellers are trying to flog them off as real, vintage, working compasses. Other sellers just can’t tell the difference and will sell these second hand in good faith, believing wholeheartedly that they are the genuine article.
Next post in the series will look at the identification of the MKIII Prismatic Marching Compass and how to avoid buying a fake. A little knowledge can save you a lot of money.