Pencil detonator
Encyclopedia
A pencil detonator or time pencil is a time fuze
designed to be connected to a detonator
or short length of safety fuse
. They are about the same size and shape as a pencil
, hence the name. They were introduced during World War II
.
(or in later versions aluminium
) tube, with a copper
section at one end which contained a glass vial
of cupric chloride
(the liquid was widely and erroneously reported to be sulfuric acid
), while beneath the vial was a spring-loaded striker
under tension and held in place by a thin metal wire. The timer is started by crushing the copper section of the tube to break the vial of cupric chloride which then slowly eats through the wire holding back the striker. The striker then flies down the hollow centre of the detonator and hits the percussion cap
at the other end of the detonator.
No. 10 delay switches had delays ranging from 10 minutes to 24 hours and were accurate to within plus or minus 2 or 3 minutes in an hour's delay and plus or minus an hour in a 12-hour delay, though environmental conditions could affect this. The switches were typically issued in packs of 5, all the switches in a pack having the same delay. In use, two switches with the same delay (from different packs if possible) would be placed in the explosive charge in case one switch failed.
Using a No. 10 delay switch:
"Crush the end of the thin copper
tube containing the cupric chloride with pliers
, or under the heel of your boot. There is no need to crush the end of the tube completely flat. All that is required is to crush and dent the tube sufficiently to break the glass vial, thereby releasing the liquid contained within. Check the inspection hole next to the brass safety strip. If the inspection hole is unobstructed (i.e. it is possible to see right through to the other side) then the countdown has started and the brass safety strip (holding back the striker) should be removed and discarded. However, if the inspection hole is obstructed (before the safety strip is removed) the striker has been released so the pencil detonator should be discarded and another one selected. The final step is to insert the end of the pencil which has the actual detonator
fitted into the explosives and leave the area."
of MD1
known as the "Lead Delay switch" or officially "Switch, No. 9, L Delay". Instead of relying on the action of acid on metal which was subject to temperature variation, it used a piece of metal under stress - the metal in question being a lead
alloy that was extremely affected by mechanical creep
. A piece of this lead was notched to a set diameter, the diameter setting the time delay. When the starting pin was removed, this wire was placed under tension by the spring-loaded striker, and began to gradually stretch. After certain time it would snap at the notch and allow the striker to hit the percussion cap.
The delay could be set from a matter of minutes to hours. Manufacture was entirely by MD1.
Generally speaking L-delays were slightly less reliable and had shorter delays, but were more reliable underwater (if a No. 10 fuze developed a leak, water would dilute the corrosive liquid and increase the delay or stop the fuze from working).
) which had an unusually fast burn rate of over 10 metres per second.
of the corrosive liquid in the vial. It is widely reported that the wire thickness varied also, but in fact all used the same diameter of wire. The time delay of a No. 9 is determined solely by the thickness of the notch in the wire, the spring
tension
, and the temperature
. Pencil detonators could be used with any explosive provided a suitable booster
was attached. However, plastic explosive
s (which did not require a booster) were particularly useful during the sabotage missions in which they were often employed. There were also a number of special charges issued with a time pencil already incorporated e.g. some types of limpet mines.
After being activated a pencil detonator is silent in operation. It does not fizz or make any other noise. However, unlike clockwork
timers, pencil detonators only give approximate time delays. For example, a 2 hour pencil detonator might be accurate to plus or minus 5 minutes, whereas the version offering a 6 hour delay could have a precision
of plus or minus 15 minutes. Both No. 9 and No. 10 delays were also significantly affected by the ambient temperature, and were issued along with a chart of temperature corrections—but no thermometer
. For example, a pencil detonator designed to fire 24 hours after being activated could in reality give a 30 hour delay - if the weather was very cold. Similarly, during hot weather pencil detonators designed to fire after a 12 hour delay could in reality trigger detonation within 10 hours. The main virtue of pencil detonators is their small size and light weight, plus the fact that they are very quick and easy to use. These are important points during covert operations.
Pencil detonators saw heavy use during the Second World War by the Special Air Service
, Special Operations Executive
and groups such as the French Resistance
. A number of pencil detonators were used to detonate the massive amatol
charge hidden inside HMS Campbeltown
during the St Nazaire Raid
of 1942. The ship exploded over an hour later than anticipated.
Approximately 12 million pencil detonators were produced in Britain during the war. However, in recent years they have been superseded by electronic timers which are more accurate and provide much longer delay times. Interestingly, pencil detonators are by nature completely immune to detection or jamming via electronic countermeasures
or EMP
devices - unlike electronic timers. For this reason they may still have applicability in special situations.
The briefcase bomb used in the July 20 plot
used a captured British pencil detonator inserted into a block of British plastic explosives weighing approximately two pounds. The bomb was set to 30 minutes and detonated as planned, but Hitler survived with minor injuries. Stauffenberg
could not prepare the second block, though. He got rid of it while driving through the forest to the airfield. His driver, Leutnant Erich Kretz, reported seeing Werner von Haeften
throw something into the woods in his mirror.http://www.filmreporter.de/?cat=1&cnc_id=19736
Fuze
Fuze Beverage, commercially referred to as just Fuze , is a manufacturer of teas and non-carbonated fruit drinks enriched with vitamins. Currently the brand consists of five vitamin-infused lines: Slenderize, Refresh, Tea, Defensify, and Vitalize...
designed to be connected to a detonator
Detonator
A detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....
or short length of safety fuse
Fuse (explosives)
In an explosive, pyrotechnic device or military munition, a fuse is the part of the device that initiates function. In common usage, the word fuse is used indiscriminately...
. They are about the same size and shape as a pencil
Pencil
A pencil is a writing implement or art medium usually constructed of a narrow, solid pigment core inside a protective casing. The case prevents the core from breaking, and also from marking the user’s hand during use....
, hence the name. They were introduced during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
No. 10 delay switch
One type, the British No. 10 delay switch (official name, "Switch, No. 10, Delay" and often referred to as a "timing pencil"), was made of a brassBrass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
(or in later versions aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
) tube, with a copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
section at one end which contained a glass vial
Vial
A vial is a relatively small glass vessel or bottle, especially used to store medication as liquids, powders or in other forms like capsules. They can also be sample vessels; e.g., for use in autosampler devices in analytical chromatography.The glass can be colourless or coloured, clear or amber...
of cupric chloride
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....
(the liquid was widely and erroneously reported to be sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
), while beneath the vial was a spring-loaded striker
Firing pin
A firing pin or striker is part of the firing mechanism used in a firearm or explosive device e.g. an M14 landmine or bomb fuze. Firing pins may take many forms, though the types used in landmines, bombs, grenade fuzes or other single-use devices generally have a sharpened point...
under tension and held in place by a thin metal wire. The timer is started by crushing the copper section of the tube to break the vial of cupric chloride which then slowly eats through the wire holding back the striker. The striker then flies down the hollow centre of the detonator and hits the percussion cap
Percussion cap
The percussion cap, introduced around 1830, was the crucial invention that enabled muzzleloading firearms to fire reliably in any weather.Before this development, firearms used flintlock ignition systems which produced flint-on-steel sparks to ignite a pan of priming powder and thereby fire the...
at the other end of the detonator.
No. 10 delay switches had delays ranging from 10 minutes to 24 hours and were accurate to within plus or minus 2 or 3 minutes in an hour's delay and plus or minus an hour in a 12-hour delay, though environmental conditions could affect this. The switches were typically issued in packs of 5, all the switches in a pack having the same delay. In use, two switches with the same delay (from different packs if possible) would be placed in the explosive charge in case one switch failed.
Using a No. 10 delay switch:
"Crush the end of the thin copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
tube containing the cupric chloride with pliers
Pliers
Pliers are a hand tool used to hold objects firmly, for bending, or physical compression. Generally, pliers consist of a pair of metal first-class levers joined at a fulcrum positioned closer to one end of the levers, creating short jaws on one side of the fulcrum, and longer handles on the other...
, or under the heel of your boot. There is no need to crush the end of the tube completely flat. All that is required is to crush and dent the tube sufficiently to break the glass vial, thereby releasing the liquid contained within. Check the inspection hole next to the brass safety strip. If the inspection hole is unobstructed (i.e. it is possible to see right through to the other side) then the countdown has started and the brass safety strip (holding back the striker) should be removed and discarded. However, if the inspection hole is obstructed (before the safety strip is removed) the striker has been released so the pencil detonator should be discarded and another one selected. The final step is to insert the end of the pencil which has the actual detonator
Detonator
A detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....
fitted into the explosives and leave the area."
L-Delay ("No. 9 delay switch")
Another, superior type, was developed by Millis JefferisMillis Jefferis
Major-General Sir Millis Rowland Jefferis KBE MC was, during the Second World War, the founder of a special unit of the British Ministry of Supply which developed unusual weapons.-Early career:...
of MD1
MD1
Ministry of Defence 1 , also known as "Churchill's Toyshop" was a British weapon research and development organisation of the Second World War....
known as the "Lead Delay switch" or officially "Switch, No. 9, L Delay". Instead of relying on the action of acid on metal which was subject to temperature variation, it used a piece of metal under stress - the metal in question being a lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
alloy that was extremely affected by mechanical creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
. A piece of this lead was notched to a set diameter, the diameter setting the time delay. When the starting pin was removed, this wire was placed under tension by the spring-loaded striker, and began to gradually stretch. After certain time it would snap at the notch and allow the striker to hit the percussion cap.
The delay could be set from a matter of minutes to hours. Manufacture was entirely by MD1.
Generally speaking L-delays were slightly less reliable and had shorter delays, but were more reliable underwater (if a No. 10 fuze developed a leak, water would dilute the corrosive liquid and increase the delay or stop the fuze from working).
Percussion igniter
Another type of time pencil had a percussion cap but no detonator attached. Instead there was a crimping attachment at one end to allow pyrotechnic fuse to be crimped on. When a time pencil of this type fired, it would light the fuse which would burn towards a detonator crimped onto the other end. Because standard safety fuse burns at around half a metre per minute, it is not practicable to provide delays of more than a few minutes in this way. It was also possible to connect a pencil detonator to so-called "instantaneous fuse" (not to be confused with detonating cordDetonating cord
Detonating cord is a thin, flexible plastic tube filled with PETN . With the PETN exploding at a rate of approximately 4 miles per second, any common length of det cord appears to explode instantaneously...
) which had an unusually fast burn rate of over 10 metres per second.
Characteristics
Pencil detonators are colour coded to indicate the nominal time delay, which can range from 10 minutes through to 24 hours. No. 10 delays were normally issued in a tin of 5, all of the same delay, while L-delays were issued in a larger tin which included a mixture of different delays to suit a variety of operations. The time delay of a No. 10 varies according to the concentrationConcentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of the corrosive liquid in the vial. It is widely reported that the wire thickness varied also, but in fact all used the same diameter of wire. The time delay of a No. 9 is determined solely by the thickness of the notch in the wire, the spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
tension
Tension (mechanics)
In physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is the magnitude of a force, it is measured in newtons and is always measured parallel to the string on which it applies...
, and the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. Pencil detonators could be used with any explosive provided a suitable booster
Explosive booster
An explosive booster acts as a bridge between a low energy explosive and a low sensitivity explosive such as TNT. It increases the explosive shockwave from an initiating explosive to the degree sufficient to detonate the secondary charge.Unlike C4 plastic explosive, not all explosives can be...
was attached. However, plastic explosive
Plastic explosive
Plastic explosive is a specialised form of explosive material. It is a soft and hand moldable solid material. Plastic explosives are properly known as putty explosives within the field of explosives engineering....
s (which did not require a booster) were particularly useful during the sabotage missions in which they were often employed. There were also a number of special charges issued with a time pencil already incorporated e.g. some types of limpet mines.
After being activated a pencil detonator is silent in operation. It does not fizz or make any other noise. However, unlike clockwork
Clockwork
A clockwork is the inner workings of either a mechanical clock or a device that operates in a similar fashion. Specifically, the term refers to a mechanical device utilizing a complex series of gears....
timers, pencil detonators only give approximate time delays. For example, a 2 hour pencil detonator might be accurate to plus or minus 5 minutes, whereas the version offering a 6 hour delay could have a precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
of plus or minus 15 minutes. Both No. 9 and No. 10 delays were also significantly affected by the ambient temperature, and were issued along with a chart of temperature corrections—but no thermometer
Thermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...
. For example, a pencil detonator designed to fire 24 hours after being activated could in reality give a 30 hour delay - if the weather was very cold. Similarly, during hot weather pencil detonators designed to fire after a 12 hour delay could in reality trigger detonation within 10 hours. The main virtue of pencil detonators is their small size and light weight, plus the fact that they are very quick and easy to use. These are important points during covert operations.
Usage
For very high value targets it is recommended that two pencil detonators from different batches are used together. That way if one detonator fails the other will almost surely blow the charge. Note that if both detonators were going to work, the explosion will occur at the minimum of the two times; thus this method will also slightly reduce the average delay.Pencil detonators saw heavy use during the Second World War by the Special Air Service
Special Air Service
Special Air Service or SAS is a corps of the British Army constituted on 31 May 1950. They are part of the United Kingdom Special Forces and have served as a model for the special forces of many other countries all over the world...
, Special Operations Executive
Special Operations Executive
The Special Operations Executive was a World War II organisation of the United Kingdom. It was officially formed by Prime Minister Winston Churchill and Minister of Economic Warfare Hugh Dalton on 22 July 1940, to conduct guerrilla warfare against the Axis powers and to instruct and aid local...
and groups such as the French Resistance
French Resistance
The French Resistance is the name used to denote the collection of French resistance movements that fought against the Nazi German occupation of France and against the collaborationist Vichy régime during World War II...
. A number of pencil detonators were used to detonate the massive amatol
Amatol
Amatol is a highly explosive material made from a mixture of TNT and ammonium nitrate. Its name originates from the words ammonium and toluene...
charge hidden inside HMS Campbeltown
HMS Campbeltown (I42)
HMS Campbeltown was a "Town"-class destroyer of the Royal Navy during the Second World War. She was originally an American destroyer , and, like many other obsolescent U.S. Navy destroyers, she was transferred to the Royal Navy in 1940 as part of the Destroyers for Bases Agreement. Campbeltown...
during the St Nazaire Raid
St. Nazaire Raid
The St Nazaire Raid or Operation Chariot was a successful British amphibious attack on the heavily defended Normandie dry dock at St Nazaire in German-occupied France during the Second World War. The operation was undertaken by the Royal Navy and British Commandos under the auspices of Combined...
of 1942. The ship exploded over an hour later than anticipated.
Approximately 12 million pencil detonators were produced in Britain during the war. However, in recent years they have been superseded by electronic timers which are more accurate and provide much longer delay times. Interestingly, pencil detonators are by nature completely immune to detection or jamming via electronic countermeasures
Electronic countermeasures
An electronic countermeasure is an electrical or electronic device designed to trick or deceive radar, sonar or other detection systems, like infrared or lasers. It may be used both offensively and defensively to deny targeting information to an enemy...
or EMP
Electromagnetic pulse
An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
devices - unlike electronic timers. For this reason they may still have applicability in special situations.
The briefcase bomb used in the July 20 plot
July 20 Plot
On 20 July 1944, an attempt was made to assassinate Adolf Hitler, Führer of the Third Reich, inside his Wolf's Lair field headquarters near Rastenburg, East Prussia. The plot was the culmination of the efforts of several groups in the German Resistance to overthrow the Nazi-led German government...
used a captured British pencil detonator inserted into a block of British plastic explosives weighing approximately two pounds. The bomb was set to 30 minutes and detonated as planned, but Hitler survived with minor injuries. Stauffenberg
Claus Schenk Graf von Stauffenberg
Claus Philipp Maria Justinian Schenk Graf von Stauffenberg commonly referred to as Claus Schenk Graf von Stauffenberg was a German army officer and Catholic aristocrat who was one of the leading members of the failed 20 July plot of 1944 to assassinate Adolf Hitler and remove the Nazi Party from...
could not prepare the second block, though. He got rid of it while driving through the forest to the airfield. His driver, Leutnant Erich Kretz, reported seeing Werner von Haeften
Werner von Haeften
- See also :* German Resistance* List of members of the July 20 plot...
throw something into the woods in his mirror.http://www.filmreporter.de/?cat=1&cnc_id=19736
See also
- Percussion capPercussion capThe percussion cap, introduced around 1830, was the crucial invention that enabled muzzleloading firearms to fire reliably in any weather.Before this development, firearms used flintlock ignition systems which produced flint-on-steel sparks to ignite a pan of priming powder and thereby fire the...
- Blasting capBlasting capA blasting cap is a small sensitive primary explosive device generally used to detonate a larger, more powerful and less sensitive secondary explosive such as TNT, dynamite, or plastic explosive....
- Exploding-bridgewire detonatorExploding-bridgewire detonatorThe exploding-bridgewire detonator is a type of detonator used to initiate the detonation reaction in explosive materials, similar to a blasting cap in that it is fired using an electric current...
- Slapper detonatorSlapper detonatorA slapper detonator, also called exploding foil initiator , is a relatively recent kind of a detonator developed in Lawrence Livermore National Laboratory...
- PrimacordPrimacordPrimacord is a registered trademark of detonating cord used in blasting, originally manufactured by the Ensign-Bickford Company. Ensign Bickford sold the trademark to Dyno Nobel Inc in 2003 and it is now manufactured in Graham, Kentucky...
- DetonatorDetonatorA detonator is a device used to trigger an explosive device. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common....
