Pencil detonator

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 shape and dimensions as a pencil.


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 brass (or in later versions aluminium) tube, with a copper section at one end which contains a glass phial of cupric chloride (the liquid was widely and erroneously reported to be sulfuric acid), while beneath the phial was a spring-loaded striker under tension and kept in place by a thin metal wire. The timer is started by crushing the copper section of the tube to break the phial 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 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 brass safety strip holding back the striker. 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. However, if the inspection hole is obstructed, the striker has previously been released, in which case the pencil detonator should be discarded and another one selected. The final step is to insert the other end of the pencil detonator (which has the actual detonator fitted) into the explosives and leave the area.

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).

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.


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 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 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.


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. 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. For this reason they may still have applicability in special situations. As at April 2008, what appears to be a modern version of the L-delay is currently produced by Armstec Ltd [] .

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 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. The car driver, Leutnant Erich Kretz, reported about seeing Werner von Haeften throwing something in the wood in the back mirror. []

ee also

*Percussion cap
*Blasting cap
*Exploding-bridgewire detonator
*Slapper detonator

External links

* [ Schematic of a time pencil]
* [ of a time pencil] (107 kB, JPEG)]
* [ photographs of time pencils] (269 kB, GIF)
* [ Electronic fuze detection method (cannot detect pencil detonators)]

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