Time Pencils: SOE’s Delayed-Action Detonators

Introduction

The “time pencil,” formally designated as the Switch, No. 10, Delay, was one of the most critical pieces of equipment issued to the Special Operations Executive (SOE) and its allies during the Second World War [1][2]. Designed as a compact, delayed-ignition device, it enabled agents and resistance fighters to initiate demolition charges with a predetermined time lag, providing the necessary window to escape the vicinity of a target before detonation [3][4].

Development and Origins

While the Switch No. 10 became synonymous with British clandestine operations, the concept originated in Germany in 1916 [4]. The design was later adopted by Polish forces. In 1939, Brigadier Colin McVean Gubbins encountered an example of the time-pencil during a visit to Poland and brought the device back to England [4].

The potential of the device was immediately recognized by Section D—the forerunner to the SOE [4]. Under the direction of Commander A.G. Langley, the device was further developed and refined at Aston House in Hertfordshire, UK, resulting in the standardized Switch No. 10 used throughout the war [2][4]. The scale of production was immense, with the SOE producing approximately 18 million units to support global sabotage efforts [4].

Design and Technical Characteristics

The device earned its nickname due to its size and shape, which closely resembled a standard writing pencil [2]. The Mk I version was approximately 5 inches in length with a diameter of 5/16 of an inch, weighing roughly 1.5 ounces [3].

Construction

The switch consisted of a housing tube made of two primary materials: a thin copper sleeve and a thin aluminium sleeve [3]. The internal mechanism comprised:

• A Glass Vial: Contained within the copper section was a small glass vial of corrosive liquid. While often erroneously reported as sulfuric acid, the liquid was actually cupric chloride [2].
• The Striker: A spring-loaded striker was held under tension beneath the vial [2].
• The Retaining Wire: A thin metal wire held the striker in place, attached to the upper end of the striker and fastened to the top of the copper sleeve [3].
• The Base: A percussion cap and fuse adapter were crimped into the base of the switch [3].

Mechanism of Action

The timer was activated by manually crushing the copper section of the tube, which shattered the internal glass vial [2]. Once the vial broke, the cupric chloride began to chemically erode the thin metal retaining wire [2][3]. The time delay was determined by the time it took for the corrosive liquid to eat through the wire. Once the wire parted, the tensioned striker was released, firing into the percussion cap and initiating the connected detonator or safety fuse [2][3].

Operational Deployment

The Switch No. 10 was widely distributed to SOE operatives and was also utilized by the American Office of Strategic Services (OSS) [4]. Because they were small and easily concealed, they were ideal for infiltration missions.

In the field, time pencils were often delivered in small quantities via airdrops. For example, shipments to the Danish resistance included stamped tin boxes containing five time-pencils each, often paired with rolls of Bickford safety fuse and detonators to create a complete sabotage kit [4].

Limitations and Legacy

As a chemical-based timer, the Switch No. 10 was subject to environmental variables. The rate of corrosion of the retaining wire could be affected by temperature, meaning the actual delay time could vary from the theoretical timing. Despite this, the device remained the standard for delayed sabotage due to its simplicity and lack of moving mechanical parts that could jam or be detected by sound.

Today, examples of the Switch No. 10 are preserved in military collections, including the Imperial War Museums and the Museum of the British Resistance Organisation, serving as artifacts of the technical ingenuity employed in the “ungentlemanly warfare” of the 1940s [1][2].