Pencil Detonator - Characteristics

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 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 explosives (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.