Switch Number 9 – L Delay, or Lead Delay.
A time pencil that was waterproof and with timings that were more exact than the No. 10 switch was needed. A time delay switch operating on the principle that a Tellurium lead rod will stretch and break under spring tension within a certain time at a certain temperature, which can be fairly accurately calculated, was created. The breaking of the finely machined rod inside caused a spring retained striker pin to set off a percussion cap and activate the fuse or explosives connected to it. These were supplied with a tag attached stating the number of hours delay before they would set off. This was named No. 9 "Lead Delay" switch.
These were also affected by temperature. Warm meant it activated sooner, cold meant later. Although thought to work at closer to their normal rating than the chemical switch, a Special Air Service veteran said, after some premature firings, he no longer liked the L-Delay.
John K. Neuenburg