In this very rare footage we see a Free French Air Force officer, possibly training as a member of the SOE, place a limpet mine on a substantial piece of metal plate.
The mine seen in the footage is clearly much smaller than the Limpets used against ships. The Limpet mine was developed by Military Intelligence (Research) in late 1939-40. Stuart Macrae and Cecil Vandepeer Clarke developed a mine with enough magnetic strength to attach an explosive charge to the hull of a ship. The initial design seen here was quite large but the design was refined as the war went on with various types and marks. Here’s a Type II limpet, a MkIII and here is a Type 6 MkII.
The idea was that divers or saboteurs in small boats could quietly attach the mines to enemy shipping while at anchor. However, the usefulness of magnetic charges was clear and it appears that smaller versions, like that we see in the footage here, were developed for use against armoured vehicles and other substantial armoured targets.
It’s unclear from the film what the explosive charge was, how big it was or how it was laid out inside the mine but from the damaged plate displayed at the end of the footage it may have been a ring of plastic explosive held in place by the four magnets. This would blow the characteristic round hold in the plates.
Interestingly, the limpet mine seen in the film is very similar to a Japanese design, the Type 99 anti-tank mine, however, it has a different fuse design and the four magnets are blocky rather than rounded. Whether the Japanese magnetic mine influenced this design developed by SOE is unknown.
I’ve been unable to find out these mine’s designation, it may not have been given one but it does appear to be fairly well developed. In this photograph we can see that a metal plate carrier has been developed to allow a soldier to carry 4 mines on his back. Perhaps these mines were developed for a specific mission. The magnetic Clam charge, which we have covered in an earlier video, would have done a similar job for smaller task
Destroying railway infrastructure was a key mission for the Resistance groups and SOE agents active in occupied Europe. Numerous methods of damaging or destroying railways were developed, including Exploding Coal, which we have covered earlier in this series. In this 16mm colour footage, believed to have been filmed in 1940, we get an early look at the methods the SOE were developing to destroy track. The ultimate aim was to derail the locomotive and wreck the train with minimal effort and explosive.
In the footage we see two charges have been placed on the piece of track, with detcord attached to both. A soldier, with what appears to be a lever-action Winchester 94, is then seen taking aim. It seems he’s aiming at a striker board attached to ignite the detcord. He fires, we see a puff of smoke and a second later the charges detonate.
The footage then cuts to several men collecting the debris of the shattered piece of track. The track appears to have two large chunks blown out and the top edge, between the two charges, completely blown off.
Later in the war more testing was done and more refined techniques were developed. In their book SOE: The Scientific Secrets Boyce & Everett note that trials of devices and techniques for destroying railway lines carried out at Longmoor where the British Army had extensive sections of track and samples of rails used in different European countries. Trials to find the right quantity and positioning of explosive charges were carried out in late December 1943, these tests would inform later operations.
The SOE’s Descriptive Catalogue of Special Devices and Supplies includes a pair of illustrations demonstrating two methods of laying and detonating these charges. A so-called ‘French’ method with a pair of what the catalogue terms ‘Igniters, Fuze, Fog Signal, MkIA’ ahead of the charges in the direction the train was expected from. The train would crush these Fog Signals firing them and igniting a length of detcord linked to a pair of 3/4lb explosive charges fixed to the track as we see in this film.
The alternative ‘Polish’ method had the same sized and located explosive charges but placed a Fog Signal either side of the charges to ensure that no matter which direction the train came from the charges would be detonated. This method was used on single track stretches of railway. Both of these methods were rated to ‘remove about one metre of rail.’
In this photo we see a member of the French Resistance setting an explosive charge on a railway line. While likely a posed photo we do see the pair of Fog Signals which will stet the charge off. These photographs show a pair of trains reportedly derailed by explosive charges.
Boyce & Everett in their book SOE: The Scientific Secrets suggest that as many as 48,000 ‘Railway charges’, presumable a kit, were produced by the SOE. From the footage we can certainly see this method of destroying rails was effective.
Today, we’re lucky enough to have some colour footage showing the of testing of a magnetic bomb which could be attached to the petrol tank of vehicles. The footage comes courtesy of the Imperial War Museum.
From the film we can see that the bomb consisted of a small block of plastic explosive, a pair of strip magnets (or possible a horseshoe-shaped magnet) and a Switch No.10 time pencil delay detonator. The explosive block itself looks to be slightly smaller than the SOE’s standard 1.5lb charge.
In the film we see the bomb placed on the boot (or trunk) of a saloon car before various civilians and a corporal experiment with various ways of covertly attaching the bomb to the underside of the car. At one point the corporal allows himself to be dragged along behind the vehicle before making his escape.
Luckily the 16mm footage, filmed by Major Cecil Clarke, also shows us the effect of the explosive charge mounted on a petrol tank full of fuel. According to the details listed for the film by the Imperial War Museum the footage was filmed in 1940, at SOE Station XVII, located at Brickendonbury House in Hertfordshire.
This configuration of the bomb doesn’t appear in the Special Operations Executive’s Descriptive Catalogue of Special Devices and Supplies published in 1944. However, Colonel Leslie Wood, Station XII’s commanding officer, described the demonstration put on during a visit by Brigadier Robert Laycock of the Commandos and William Donovan, the head of the American OSS in June 1942. One of the scheduled demonstrations was the “Effect of small ‘magnet’ charge of explosive on petrol tank of car.”
It appears that this ad hoc magnet charge evolved into ‘the Clam’, which was a smaller, version of the magnetic Limpet mine. The Clam evolved through a number of marks with the MkI having a stamped sheet metal casing and the later MkIII using a bakelite, plastic casing. Both were made up of a plastic explosive charge inside a rectangular, rounded case with a pair of magnets at either end. They were detonated by either a Time Pencil or an L Delay fuse attached to a No.27 detonator. The MkIII had 8oz (226g) of high explosive filler, such as TNT/Tetryl 55/45.
While unlike the larger Limpet they weren’t developed for under water use but the Clam could be mount onto any vaguely flat magnetic surface including engine blocks, fuel tanks, crank cases, cylinder blocks, rail tracks and steel plate.
At just 5.75” x 2.75” x 1.5” they were easily concealable, could be carried in a pocket and were non-descript enough not to draw attention. An estimated 68,000 Clams were made under supervision at Aston House according to Des Turner’s book on Station XII.
We’re lucky enough to have some unique colour footage showing the of testing of some of these explosive devices and in this article we will examine an incendiary-filed case.
In this piece of 16mm colour footage, filmed in 1940 by Captain Cecil V. Clarke, we see what appears to be an attaché case containing three medium-sized bottles, which likely contains a mix of petrol and paraffin or some white phosphorus, prepared for testing at the bomb range at Brickendonbury in Hertfordshire, a Special Operations Executive training and research centre codenamed Station XVII. It’s believed that these films may have been produced as teaching aids for the agents trained at Station XVII and this film may have been shown during a lecture.
While incendiary briefcases, attaché cases and even suitcases are listed in the 1944 SOE Descriptive Catalogue of Special Devices and Supplies they were quite different from this case. They were primarily designed for the quick destruction of documents and items carried inside them. They used sheets of potassium nitrate to burn the case’s contents.
The incendiary case seen in this footage on the other hand appears to be designed to be clandestinely placed and detonated with a delay fuse, to set nearby flammable objects on fire. What was described as a ‘Delayed Action Incendiary’.
In this footage of another separate test we get an idea of the destructive capability of just one of the bottles.
It’s possible that this incendiary case was a proof of concept test for the later cases or perhaps a demonstration of a concealed incendiary device Station XVII were working on. SOE developed a large number of bespoke explosive devices for various missions, so while this device may not have become ‘standard issue’, it may have been developed for a specific purpose.
During the Second World War Britain’s Special Operations Executive (SOE) developed a whole series of sabotage devices for use behind enemy lines. Using unique archival footage this series of short videos examines some of the weapons developed for use by SOE agents in occupied Europe. We begin the series with a look at the history and development of Explosive Coal. Explosive coal was designed to explode inside fireboxes, furnaces and coal stores hampering enemy infrastructure.
I came across this footage while doing some research in the Imperial War Museum’s online catalogue. This piece of 16mm film was filmed by Cecil Vandepeer Clarke, a British engineer and sabotage expert who was a member of the Special Operations Executive and worked at a number of weapon research and development centres including MD1 at Whitchurch and SOE Station XII at Aston.
SOE or Special Operations Executive were a clandestine force tasked with conducting irregular warfare behind enemy lines including sabotage, assassination, intelligence gathering an small scale raiding. One of the sabotage methods developed was introducing an explosive charge into the boiler firebox of a ship or a locomotive or a power station or factory’s furnace. This achieved by disguising the explosive as either a piece of fuel like coal or wood or even as a dead rat – which might be tossed into a firebox or furnace to be disposed of.
The idea of ‘Explosive Coal’ wasn’t new. The idea originated from the US Civil War, when Confederate Captain Thomas Edgeworth Courtenay designed a piece of cast iron, with a cavity which could be packed with gun powder, that looked like a lump of coal. The Courtenay described them as ‘Coal Torpedoes’, their aim was to damage a steam ship’s boiler enough to cause a catastrophic secondary explosion. While several vessels may have been damaged or sunk by these Coal Torpedoes, the claims are difficult to confirm.
It seems the idea of a coal bomb was resurrected in 1940 and initially a ‘Coal Borer’ was developed and available for use in theatre by mid-1940. The borer could be used by agents to make holes in lumps of coal which could be filled by plastic explosive and a detonator. This was soon superseded by an Explosive Coal Kit which included moulded fake coal and paints to allow agents to match the colour of local coal. The kit included instructions on how to prepare and use the coal bomb.
Arthur Christie, a lab assistant at Station XII, is quoted at length in Des Turner’s book on Station XII. Christie remembered being asked to drill large holes in some coal:
“Another task was collecting the biggest lumps of coal that I could find in the storeroom and taking them to the lab. I had no idea what they wanted them for; it was seldom explained to me and, when it was, it was often as clear as mud. My instructions were to try to drill a large hole in each piece of coal without shattering it. I tried with a brace and a six-inch long tube that had a serrated end. I found that, if too much pressure was applied, the coal would disintegrate. I thought, I wonder what the hell they want this for? Don’t ask, just do it, and I did manage to drill three lumps of coal. I placed the drilled coal on the table of the MI room and set off for the officers’ dining room to inform the CO that I had been successful. I was told to insert about a quarter of a pound of PE and a detonator into the hole and glue the coal dust back over it. The mud in my brain now began to clear. The lump of coal could be placed in the coal tender of a locomotive and find its way into the firebox, or perhaps into the furnace of a factory. Later the PE was dyed black, which was better than using coal dust and glue. This idea led to plastic explosive being moulded into a multitude of objects and colours to fool the enemy.”
Frederic Boyce & David Everett, in their book SOE: The Scientific Secrets, credit Station XV with the development of a moulded clam-shell design using dyed Herculite plaster and coated with real coal dust. A photograph of this can be seen in the SOE’s Descriptive Catalogue of Special Devices and Supplies, along with ‘Explosive Wood’, or as it was officially known ‘Wooden Logs, Explosive’.
Eventually this was replaced by a bomb based around a charge in a metal casing that allowed liquid plaster to be poured around it, simplifying production and removing any sign of a seam. The coal bombs were detonated by a No.27 Detonator to which either a match headed safety fuse or a time delay fuse was attached.
Once the danger of coal bombs was discovered by the enemy it was also believed that they would have considerable a psychological impact and also cause the enemy to expend considerable resources on protecting and checking coal supplies.
The ‘Explosive Coal’ we see in the footage appears to actually be an incendiary bomb, producing a large amount of flame and heat. This would have been ineffective in a boiler but with a time delay or other sort of fuse it may have been very effective in causing a coal bunker fire aboard a ship, in a factory store, at a coal depot or in a locomotive’s coal tender. Coal fires are extremely difficult to contain and put out.
How effective Explosive Coal was is unclear but it is believed that coal bombs were used by both the SOE and their American counterparts the OSS. Boyce & Everett estimate that about 3.5 tons of explosive coal was made between 1941 and 1945. I’m unsure how many of these were explosive and how many were incendiary, like that seen in the footage here, but it’s a fascinating asymmetric method of targeting enemy infrastructure at the most basic level.