Pencil detonator

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.

One type, the British Number Ten 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 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 was started by crushing the copper section of the tube to break the vial of cupric chloride, which then began to slowly erode the wire holding back the striker. When the wire eventually parted, the striker was propelled down the hollow centre of the detonator, hitting the percussion cap at the other end of the detonator.

Number ten delay switches had delays ranging from 10 minutes to 24 hours and were accurate to within plus or minus two or three 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 five, 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.

"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."

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Wikipedia