Switch Number 9 – L Delay, or Lead Delay.
A time delay switch operating on the principle that a lead wire will stretch and break within a certain time at a certain temperature, which can be accurately calculated. The breaking of the wire inside caused a spring retained striker pin to set off a percussion cap and activate the explosives connected to it. These were supplied with a tag attached stating the number of hours delay before they would set off.
A time pencil that was waterproof and with timings that were more exact was needed. The No. 9 "Lead Delay" switch was invented. These were also affected by temperature. Warm meant it activated sooner, cold meant later. An SAS vet said after some premature firings he no longer liked the L-Delay.
John K. Neuenburg