Pencil detonator

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Introduced during World War II, a pencil detonator or time pencil is a chemically activated time fuze designed to be connected to a detonator or short length of safety fuse. Pencil detonators are so-called because they have approximately the same dimensions as a pencil.

1 Types
2 Characteristics
3 Usage
4 Usage In The 1944 Plot To Assassinate Hitler
5 External links

[edit] Types

[edit] No. 10 delay switch

One type, the British "Switch, No. 10, Delay", is made entirely of aluminium (or brass in early versions), apart from a copper section at one end which contains a glass vial of a green corrosive liquid. (The liquid was widely reported as sulphuric acid, but was actually cupric chloride, which corrodes iron through a redox reaction.) Underneath the vial is a spring-loaded striker under tension, held back by a thin steel wire. After the liquid is released by crushing the copper tube it slowly eats through the wire holding back the spring-loaded striker. The centre of the pencil detonator is hollow to allow the freed striker to fly down it and hit the percussion cap on the detonator at the other end.

Using a pencil detonator is a straightforward process:-

Crush the end of the thin copper tube containing acid 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. Then remove and discard the safety pin holding back the striker. Finally, insert the other end of the pencil detonator into the explosives. Leave the area.

[edit] No. 9 delay switch (L-delay)

Another type, known as the "Switch, No. 9, L Delay", contained a thin, notched wire of a special lead alloy that was extremely affected by mechanical creep. When the starting pin was removed, this wire was placed under tension by the spring loaded striker, and began to gradually stretch. After a delay it would snap at the notch and allow the striker to hit the percussion cap. 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, it would dilute the corrosive liquid and increase the delay, or stop working altogether).

[edit] 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.

[edit] 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 concentration of the corrosive liquid in the vial. (It is widely reported that the wire thickness varied also, but in fact all used exactly the same 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 primer was fitted, however plastic explosives were particularly useful with the sabotage missions for which they were often employed. There were also a number of special charges issued with a time pencil already built in, such as 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. The main virtue of pencil detonators is their small size, plus the fact that they are very quick and easy to use. These are important points during covert operations.

[edit] 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 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 Operations Executive and groups such as the French Resistance. 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 immune to detection or jamming via electronic countermeasures because they use a chemical time fuze. For this reason they may still have applicability in special situations.

[edit] Usage In The 1944 Plot To Assassinate Hitler

The briefcase bomb used in the July 20 plot used a captured British pencil detonator inserted into a block of British plastic explosives weighing two pounds. The bomb detonated as planned, but Hitler survived with minor injuries.