Explosives and Booby Traps by CART CIO Peter Antill.

This page was last updated on 28/3/16

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The Auxiliary Units, as well as being the first to be issued various ranged weapons such as the Thompson and the PIAT, were also the first to be issued with Sticky Bombs (also known as the ST Grenade or No. 74 Grenade and designed by Major Millis Jefferis) and phosphorous hand-grenades and before anyone else.

They were given one of Military Intelligence (Research)'s uglier weapons, the Switch No. 8 AP – or as it was later know to soldiers in the Western Desert, the open 'castrator'. This booby-trap device was issued to the Resistance in parts of Scotland, but because of the way it worked it was never actually planted in training. A metal tube painted matt black, about the size of a fountain pen, the stick pencil contained a firing pin and a spring shaped like an umbrella catch. It was plugged vertically into the ground and a cartridge was inserted into it, nose upward, with the bullet protruding slightly out of the earth. The weight of a man’s foot on the bullets tip was enough to cause the spring to release the firing pin. They also received the special tyre-bursting mines, in this case special hollow bronze castings, disguised as lumps of coal or horse manure, being able to hold about at least two ounces of HE and a detonator. Read more about 'Churchill's Toyshop'.

In many Aux Unit areas, a plentiful supply of explosives and the accessories necessary for sabotage and booby-trapping were available from mid-July 1940. The volume and variety of said explosive were available from a variety of sources, including the War Office, Section D of MI6 (the SIS) and commercial suppliers and included Nobel 808, dynamite, ammonal, gelignite and plastic explosive (also known as plastique). It is worth noting that a few pounds of HE in the wrong hands can destroy entire buildings and kill dozens of people – the average Aux Unit having upwards of half-a-ton. For example, Reg Sennet, the CO for the Dengie Group of Aux Patrols, gave up after waiting for twenty years for the Army to come and collect the ordnance his patrols had left behind in his milking shed after stand-down. He eventually put aside his commitments under the Official Secrets Act and told the local Police, who in turn called the Army. They retrieved:

• 14,738 rounds of ammunition;
• 1,205lbs of explosives;
• 3,742 feet of delayed action fusing;
• 930 feet of safety fuse;
• 144 time pencils;
• 1,207 L-Delay switches;
• 1,271 detonators;
• 719 booby-trap switches;
• 314 paraffin bombs;
• 131 fog signals;
• 121 smoke bombs;
• Thirty-six slabs of guncotton;
• Thirty-three time pencils (click here for more information) and booby-trap switches attached to made-up charges

Grenades

The Aux Units were issued with three standard types of hand grenade.

	The first, was the No. 36 Grenade, also known as the Mills Bomb. Each Aux Unit had at least one case load. It had a maximum danger range of about 50 yards but could only be thrown a maximum of 35 yards!
	The Grenade, Hand, Anti-Tank No. 74, commonly known as the S.T. Grenade or sticky bomb, the grenade consisted of a glass sphere containing an explosive made of nitro-glycerine covered in a powerful adhesive, and surrounded by a sheet-metal casing. When the user pulled a pin on the handle of the grenade, the casing would fall away and expose the sphere; another pin would activate the firing mechanism, and the user would then attempt to attach the grenade to an enemy tank.
	As already mentioned, the No. 76 Grenade was again standard issue, and was also known as the A.W. or S.I.P. (Self-Igniting Phosphorous) grenades. These were contained within a strong glass bottle for safety and sealed with a crown stopper, which if it was green, enabled it to be fired from the Home Guard's Northover projector. Developed at Churchill's 'Toyshop'.
	The Aux Units had the No. 77 smoke grenade. Even though it was a smoke grenade, there were precautions to take when using it, as it was essentially a tin with a screw down lid, containing white phosphorous and a detonator. Such grenades should be kept away from other explosives and the user when it bursts as small amounts of phosphorous are thrown out and could cause burns. The grenade would produce thick white smoke for about thirty seconds.

Explosives Pack

Finally, it is now known that most, if not all Aux Units were issued with a standard 'explosives pack', the contents of which was described in an appendix to a secret letter (Reference: AU/B/1/9a), dated 14 July 1944. It listed these contents:

  Appendix "B" to Secret letter
  AU/B/1/9a dated 14 Jul 1944.

  Contents of One Aux.Unit Mark II.

24 Copper Tube Igniters.
1 Crimping Tool.
6 Striker Boards.
1 Tube Vaseling.
1 Spool Trip Wire .032"
3 Spools Trap Wire .014"
8 Coils Tape
12 Pocket Time Incendiaries (either Red, White or Green markings)
20 1-hr. Lead Delays. (Developed at Churchill's 'Toyshop'. )
50 3-hr. Lead Delays. (Developed at Churchill's 'Toyshop'. )
50' Instantaneous* Fuze (Orange Line)
240 ft. Cordtex.
100 Detonators (Nos. 8 or 27)
20 lbs. Explosive. (Nobels 808, Polar Gelignite or Plastic)
1 Sandbag.
6 Pull Switches.
3 Pressure Switches. (Developed at Churchill's 'Toyshop'. )
48 ft. Safety Fuze Mk. II Bickford.
20 C.E.Primers. (Two tins of 10 each)
24 Tubs, Fuze, Sealing, in those Aux.units where the
  fuze is not packed in tins.

Notes.

1. Aux.Units Mk.II held in the Northumberland Area may be deficient of Copper Tube Igniters.

2. A large number of Aux.Units Mk.II will be deficient of 3-hr.L.Delays. Exact details are not obtainable but of the 360,000 which should be in circulation only 107,000 are actually contained in the Aux.Units, enumerated in Appendix "B".

3. A large proportion of the Aux.Units Mk.II will contain Magnets either Large or Small type. The number of these in each Aux.Unit will be about 6.

SWITCHES  

Prepared by Aux Researcher Matthew Gibbs.

One of the main strategies for the Auxiliary Units Operational Patrols was the destruction of enemy supplies and equipment. Much of their training related therefore to the carrying out of this aim. Explosives’ training took place with the Patrols and the Scout Sections, and was also concentrated on at Coleshill. At one typical weekend course at Coleshill House in 1941 Explosives training was the subject of a morning lecture from 0830 – 0925, then a practical session from 0930 – 1025 plus a further 55-min session on the Mills Grenade. That night was a practical patrol exercise that lasted at least 90 minutes where instructors would assess the patrols effectiveness. The following morning was a further hour slot for Explosives work. Quite a lot of information obviously relating to safety was provided, along with the strategies for the best places to attack and how to accomplish that.

Explosive devices provided to the Auxiliary Units

The explosives devices used by the Auxiliers can be simply divided into three areas.
1 The device used to set off or trigger the explosive material – usually known as a Switch
2 The device or material used to convey the trigger to the explosive material, usually a Wire, Line or Cord.
3 The type of explosive material selected for use.

Switches

The supply of these devices mushroomed with the outbreak of war, and from 1938 more serious research and various groups including the British Secret Service and Military Intelligence [Research] Branch did work on them. They came up with a whole host of different devices to be used in various ways to set off explosives for sabotage etc. More of these came along with the setting up of the Special Operations Executive who took over a lot of research and development on them. More information on the background can be found in books on SOE.

The switch devices were numbered and also had a practical descriptive name, and are listed below:

Switch Number 1 Mk1 – Pull Switch. Attached to something like a trip wire, it operated when a pin on the switch was pulled. This allowed a striker pin to hit a percussion cap that triggered the fuse wire attached to it.

Switch Number 2 Mark 1 – Pressure Switch. Placed under something or disguised with camouflage it operated when a sufficient weight pressed on the top. This sheared a metal pin attached to the spring-operated striker, which then triggered the explosives connected with it.

Switch Number 3 Mark 1 – Release Switch. Placed inside or under something, this switch operated in reverse to the Pressure switch. When a weight was removed from it, or the closed part of the device released, for example the opening of a tin, this triggered the explosives connected to it. Often this was used by the army for booby traps.

Switch Number 4 Mark 1 – Pull Switch – As per number 1 but simpler in operation.

Switch Number 5 Mark 1 – Pressure Switch, operated as per number 2 but a different design.

Switch Number 6 Mark 1 – Release Switch, operated as per Number 3 but a smaller, compact design, developed by SOE research for better concealment or for applications requiring a smaller device.

Switch Number 8 AP – Anti Personnel switch, designed to be operated by weight and fired a pointed bullet up out of the ground into the foot of someone who trod on it.

Switch Number 9 (as seen above) – L Delay, or Lead Delay. A Time Delay switch operating on the principle that a lead wire will stretch and break within a certain time at a certain temperature, which can be accurately calculated. The breaking of the wire inside caused a spring retained striker pin to set of a percussion cap and activate the explosives connected to it. These were supplied with a tag attached stating the number of hour’s delay before they would set off.

Switch Number 10 – Time Pencil. A time delay switch operating on the principle that a certain strength of acid will eat through a fixed diameter copper wire within a certain time at a certain temperature. The corrosion of the wire inside the ‘pencil’ caused a spring retained striker pin to set of a percussion cap and activate the explosives connected to it. These were not as accurate as an L Delay but as they are extremely common to find it is likely that they were still fit for purpose within limits. They were supplied with a colour-coded tag that showed the number of hours delay they would provide. They also came with a temperature chart to indicate the tolerance to be prepared for. In extreme cold they would take longer to operate, and in extreme heat would work quicker.

Number 13 Switch (seen above) – a combination Pressure, Pull, or Release device, which combined the trigger variations into one Universal Switch.
A: Fog Signal Switch attached to orange line fuse for demolitions on railway lines, activated by train running over the top of the rail mounted switch.

B: Number 6 Switch

C: Number 3 Switch

D: Number 2 Switch

E: Number 5 Switch with a spike extension fitted.

Wire, Cord and Line Fuse

Under this heading comes the equipment used to transfer the action of the switch to the explosive material. Sometimes this was dispensed with in some situations. L Delays, for example, were produced with could be fitted to a detonator or device [such as a Clam Mine] and used to set off explosives directly.

Fuses, in military explosives, initiate the function of a made up charge or device. They are made up of some kind of burning material. Depending on their chemical design some will even work under water. One of the commonest is known as Safety Fuse or Bickford Fuse, named after its inventor. The ‘tube’

Safety Bickford Fuse or Fuse Wire

This was procured by the military from civilian manufacturers who supplied the mining industry, such as I.C.I. Ltd. It was produced in slow and fast burning versions. The amount of time that it took for the fuse wire to burn down was measured in seconds per feet. According to the size of the explosive charge obviously the person firing it would want to be a certain distance away and so the time was a safety factor element allowing someone to get away, or be a certain distance away before the charge fired. More often than not the fuse is fitted into a detonator [blasting cap] which detonates and causes a chain reaction explosion in the explosive material itself. Safety fuse coating is usually black, commercial versions can be orange. Other military types are highly coloured and known as cords, explained later.

The Bickford or Safety Fuse has a black powder core with a coating of ashphaltum or other waterproofing agent and a tough textile outer wrapper. It generally burns at 60 to 90 seconds per foot.

Detonating Cord or Primacord

Detonating cord differs from the above safety fuse because it is a very high speed which actually explodes rather than burns, and is used to detonate certain types of explosive without a separate detonator. Also known as Cordtex and Primacord, these are manufacturer trade names that have lapsed into common use. Its extremely high speed means it causes an almost instant detonation of the explosive once activated, over even quite long distances. For example one type is rated at 7,000 metres per second. This is not a light and then walk away fuse!