British A.C. Delay Mark I/Time Pencil

Overview

The A.C. Delay Mark I and commonly referred to as the “Time Pencil” or “Switch, No. 10, Delay,” is a chemically-timed delay initiating device developed by Great Britain during World War II for use in sabotage, demolition, and clandestine operations. The designation “A.C.” stands for Acetone-Celluloid, referring to the original delay mechanism where acetone dissolved a celluloid washer to release a spring-loaded striker. This compact, silent-operating device became one of the most important tools of Allied special operations during WWII and was instrumental in countless sabotage missions conducted by the Special Operations Executive (SOE), resistance movements, and Allied commandos.

Country/Bloc of Origin

Country of Origin: United Kingdom
Developing Organization: Section D (precursor to SOE), later Special Operations Executive (SOE) Research Station
Development Location: Station XII, Aston House, Stevenage, Hertfordshire
Key Developer: Commander A.G. Langley and SOE technical staff
Development Period: 1939-1940
Manufacturing: MD1 (Ministry of Defence 1, “Churchill’s Toyshop”) and contracted manufacturers
International Distribution: Supplied to all Allied nations, resistance movements across occupied Europe, and OSS; design shared with United States for American production
Production Volume: Approximately 12 million units produced in Britain during WWII

Ordnance Class

Type: Chemical Delay Firing Device / Time-Delay Initiator
Primary Role: Delayed initiation of demolition charges and sabotage devices
Delivery Method: Hand-emplaced
Category: Initiating and Priming Device / Clandestine Warfare Materiel
Function: Provides silent, chemically-timed delay for initiating explosive charges in sabotage and demolition operations

Ordnance Family/Nomenclature

Official British Designations:

Switch, No. 10, Delay (primary official designation)
A.C. Delay Mark I (AC = Acetone-Celluloid)
Time Pencil (common name)
Timing Pencil

Related British Variants and Family Members:

Switch, No. 10, Delay (Mark II, III, etc.): Improved production variants
Switch, No. 9, L-Delay: Lead-creep mechanical delay (Millis Jefferis design)
A.C. Delay Mark IA: Modified variant with design improvements

American Equivalent:

Firing Device, Demolition, M1, Delay (US adaptation)

Common Names and Nicknames:

Time Pencil (most common)
Timing Pencil
Delay Pencil
No. 10 Switch
SOE Pencil

Delay Time Color Codes:

Safety Strip Color	Nominal Delay Time
Black	10 minutes
Red	30 minutes
White	1 hour
Green	3 hours
Yellow	6 hours
Blue	12 hours
Purple/Grey	24 hours

Note: Color coding varied somewhat between production batches and over time

Hazards

Primary Hazards:

Corrosive Chemical Hazard: Contains glass ampoule with cupric chloride (widely misreported as sulfuric acid); causes chemical burns on skin contact
Detonation Hazard: Integrated detonator (or attached blasting cap) will initiate; blast and fragmentation hazard dependent on main charge
Unpredictable Timing: Delay times highly variable with temperature; may fire significantly earlier or later than nominal

Sensitivity Considerations:

Crush Sensitivity: Squeezing the copper tube section breaks the internal glass ampoule—avoid accidental crushing
Temperature Sensitivity: Chemical reaction rate approximately doubles for each 10°C (18°F) temperature increase
Impact Sensitivity: Rough handling may break internal ampoule or damage mechanism
Detonator Sensitivity: Percussion cap and detonator elements are impact-sensitive

Environmental Stability:

Temperature Effects:
- Cold conditions significantly extend delay times (may double or triple)
- Hot conditions significantly shorten delay times (may halve)
- Extreme cold may prevent functioning entirely
Moisture:
- Water ingress can dilute corrosive liquid, extending delay or preventing function
- Underwater use not recommended with standard No. 10 switches
- Switch No. 9 (L-Delay) was preferred for underwater applications
Storage Degradation:
- Corrosive liquid may lose potency over extended storage
- Seal integrity may degrade
- Aged devices have unpredictable performance

Special Hazards:

No Abort Capability: Once ampoule is crushed and safety strip removed, detonation is inevitable
Silent Operation: No audible countdown; no warning before detonation
Batch Variability: Even devices from the same batch may have significantly different actual delays

Danger Zones:

Determined by attached explosive charge
Minimum safe distance with standard charges: 300+ meters
Account for fragmentation hazard from target being destroyed

UXO Considerations:

Time pencils with missing safety strips are extremely dangerous
May still function after decades if corrosive liquid remains potent
Corroded or damaged devices are unpredictable
Never handle—report to EOD/military authorities
Historical examples in museum collections should be verified inert

Key Identification Features

Dimensions:

Overall Length: Approximately 5.75 inches (146mm)
Diameter: Approximately 0.5 inch (12.7mm) at narrowest; up to 0.75 inch (19mm) at coupling

Physical Characteristics:

Pencil-shaped cylindrical device (hence “time pencil”)
Brass or aluminum tube body (early versions brass; later versions aluminum for material conservation)
Copper section at one end (crush zone for ampoule)
Threaded coupling base at opposite end for detonator attachment
Color-coded safety strip/identification band wrapped around device and passing through mechanism

Color and Markings:

Body Color: Natural metal finish (brass = golden; aluminum = silver)
Safety Strip: Color-coded indicating delay time (see table above)
Markings: May include lot numbers, manufacture dates, and military broad arrow acceptance marks
Packaging: Issued in metal tins containing 5 units of same delay time

Distinctive Features:

Pencil-like overall appearance and size
Thin copper crush tube section at one end
Color-coded strip visible around body
Inspection hole near safety strip
Coupling base for attaching detonator or connecting to special charges

Material Composition:

Body tube: Brass or aluminum alloy
Crush section: Thin copper tubing
Internal components: Glass ampoule, cupric chloride solution, restraining wire, spring-loaded striker
Coupling hardware: Brass or aluminum
Detonator: Standard SOE percussion cap and detonator assembly

Fuzing Mechanisms

Operating Principle:

The British A.C. Delay operates on the principle of chemical corrosion of a restraining wire. A glass ampoule containing cupric chloride solution is housed within the copper tube section. When the copper section is crushed, the ampoule breaks, releasing the corrosive liquid. This liquid attacks a thin metal wire that holds a spring-loaded striker in the cocked position. When the wire corrodes through, the striker is driven forward into a percussion cap, which initiates the attached detonator.

Delay Mechanism Details:

Early A.C. Delay Design: Original versions used acetone to dissolve a celluloid disc
Standard No. 10 Switch: Uses cupric chloride corroding a metal wire (more reliable)
Delay Control: Different delay times achieved by varying the concentration of cupric chloride solution; more concentrated = shorter delay

Internal Components (from top to bottom):

Copper crush tube: Contains sealed glass ampoule
Glass ampoule: Contains cupric chloride solution
Restraining wire: Extends along ampoule into striker housing
Spring-loaded striker: Held under compression by restraining wire
Safety strip: Provides backup blocking of striker
Inspection hole: Allows visual check of striker position
Percussion cap: Initiated when striker is released
Detonator: Standard military detonator or special SOE detonator

Activation Sequence:

Preparation:
- Remove any protective cap from detonator end
- Attach blasting cap if not pre-attached
- Insert detonator end into explosive charge or connect to detonating cord
Activation (Crushing the Ampoule):
- Crush the copper tube section using pliers, boot heel, or firm hand pressure
- Only enough pressure to break the internal glass ampoule is required
- Do not crush completely flat
Verification:
- Check the inspection hole near the safety strip
- Clear hole: Striker still restrained; proceed
- Blocked hole: Striker has released prematurely; discard device safely
Arming (Removing Safety Strip):
- Pull and remove the color-coded safety strip
- Device is now fully armed and countdown has begun
- Leave the area immediately
Functioning:
- Cupric chloride corrodes through restraining wire (time depends on concentration and temperature)
- Wire parts; spring drives striker into percussion cap
- Percussion cap initiates detonator
- Detonator initiates main explosive charge

Critical Operating Notes:

Silent Operation: No sound, no visible indication of countdown
Irreversible: Once activated, cannot be disarmed
Temperature Critical: Always consult temperature correction charts
Redundancy: Use two devices from different batches for critical missions

Alternative Initiation: Time pencils could also be connected to safety fuse (via crimp attachment) rather than directly to a detonator, allowing the fuse to carry flame to a separate detonator.

History of Development and Use

Development Origins:

The Time Pencil was developed in response to the needs of unconventional warfare in the early stages of World War II. As Britain faced the prospect of German invasion and sought ways to strike back at occupied Europe, Section D of the Secret Intelligence Service (SIS) began developing tools for sabotage operations.

Key Development Milestones:

June 1939: Section D develops initial chemical delay mechanism
July 1940: SOE established; inherits Section D’s research
1940-1941: Development refined at Station XII (Aston House), Stevenage
1940-1941: Commander A.G. Langley leads development team
1941: Large-scale production begins
1941-1945: Approximately 12 million units produced
1942 onward: Design shared with US OSS for American production

Design Evolution:

Original A.C. Delay (Acetone-Celluloid): Used acetone dissolving a celluloid disc to release striker
Switch No. 10 (Cupric Chloride): Improved design using acid corrosion of metal wire; more reliable and controllable
Switch No. 9 (L-Delay): Mechanical creep-based delay using lead alloy under stress; developed by Millis Jefferis for underwater use
Various Marks: Production improvements throughout the war

Operational Employment:

The Time Pencil became the standard delay mechanism for SOE and Allied special operations:

SOE Operations:
- Factory sabotage across occupied Europe
- Railway destruction
- Supply depot attacks
- Ship sinking (with limpet mines)
- Resistance support operations
Notable Operations:
- St. Nazaire Raid (Operation Chariot, March 1942): Time Pencils used to detonate explosives hidden in HMS Campbeltown, destroying the Normandie dry dock (fired approximately one hour late due to temperature)
- Cockleshell Heroes (Operation Frankton, December 1942): Time Pencils used with limpet mines in attack on shipping at Bordeaux
- July 20 Plot (1944): German conspirators used captured British Time Pencils in attempt to assassinate Hitler
Resistance Movements:
- French Resistance (FFI/FTP)
- Polish Home Army (AK)
- Norwegian resistance
- Yugoslav Partisans
- Dutch resistance
- Belgian resistance
- All major European resistance movements received Time Pencils via SOE supply drops

Training and Doctrine:

SOE agents received thorough training on Time Pencil use at various “finishing schools” in Britain. Key training points included:

Always use two pencils from different batches for critical targets
Account for temperature effects on timing
Verify inspection hole is clear before removing safety strip
Allow adequate safety margin for escape
Silent operation allows use in guarded areas

Limitations Discovered in Service:

Temperature sensitivity caused several missions to have delayed or premature detonations
Batch-to-batch variation required redundant devices
Water ingress could disable No. 10 switches (led to development of No. 9 L-Delay for maritime operations)
Aged devices became unreliable

Current Status:

No longer in active military service
Replaced by electronic delay detonators and modern initiation systems
Historical examples held in museums and private collections
Design principles influenced post-war delay device development

Legacy:

The Time Pencil represents a landmark in clandestine warfare technology. Its combination of reliability, simplicity, and compact size enabled sabotage operations that would otherwise have been impossible. The psychological impact on German occupiers—never knowing when or where the next explosion would occur—was as significant as the physical damage inflicted.

Technical Specifications

Specification	Details
Overall Length	Approximately 5.75 inches (146mm)
Diameter	Approximately 0.5-0.75 inch (12.7-19mm)
Weight	Approximately 1.5-2 oz (42-57g)
Body Material	Brass or aluminum tube
Crush Section Material	Thin-walled copper
Corrosive Agent	Cupric chloride solution
Delay Mechanism	Chemical corrosion of restraining wire
Output	Standard SOE detonator (equivalent to military blasting cap)
Packaging	5 units per sealed metal tin

Delay Time Specifications:

Color Code	Nominal Delay	Accuracy
Black	10 minutes	±2-3 minutes
Red	30 minutes	±5-7 minutes
White	1 hour	±10-15 minutes
Green	3 hours	±30-45 minutes
Yellow	6 hours	±45-60 minutes
Blue	12 hours	±1-2 hours
Purple/Grey	24 hours	±2-3 hours

Temperature Correction Factors (Approximate):

Temperature	Correction Factor
100°F (38°C)	0.5x (half nominal time)
85°F (29°C)	0.7x
70°F (21°C)	1.0x (nominal)
55°F (13°C)	1.5x
40°F (4°C)	2.0x (double nominal time)
32°F (0°C)	3.0x or more

Comparison: Switch No. 10 vs. Switch No. 9 (L-Delay):

Feature	No. 10 (Chemical)	No. 9 (L-Delay)
Delay Mechanism	Acid corrosion	Lead creep
Maximum Delay	24 hours	Several hours
Reliability	Good	Slightly lower
Underwater Use	Poor	Better
Temperature Sensitivity	High	High
Manufacturing Complexity	Medium	Higher

Frequently Asked Questions

Q: Why is it called a “Time Pencil”? A: The name derives from the device’s size and shape, which closely resembles a standard writing pencil—approximately 6 inches long and half an inch in diameter. This nomenclature was deliberately non-descriptive for security purposes, avoiding terms like “bomb” or “detonator” in communications. The official designation “Switch, No. 10, Delay” was similarly opaque.

Q: What is the difference between the A.C. Delay and the Switch No. 10? A: These terms are sometimes used interchangeably, but technically: the original “A.C. Delay” used acetone dissolving a celluloid disc, while the “Switch No. 10” used cupric chloride corroding a metal wire. The Switch No. 10 mechanism proved more reliable and became the standard production type. Both operate on similar principles of chemical action releasing a spring-loaded striker.

Q: Why was the Time Pencil fired late during the St. Nazaire Raid? A: The explosives in HMS Campbeltown detonated approximately one hour later than planned. This was primarily due to the cooler-than-expected temperature inside the ship’s hull. The chemical reaction rate slows significantly in cold conditions, extending the delay time. This incident reinforced the critical importance of temperature compensation in planning sabotage operations.

Q: How did SOE agents account for temperature effects? A: Agents received temperature correction charts with their Time Pencil supplies. These charts showed multiplication factors for different temperatures. For example, at 40°F, the actual delay might be double the nominal time. Agents were trained to estimate ambient temperature and adjust their planning accordingly. For critical missions, using two pencils with slightly different nominal times from different batches provided additional insurance.

Q: Could the Germans detect Time Pencils? A: Detection was extremely difficult. Time Pencils contain no clockwork (no ticking), require no batteries (no electrical signature), and contain relatively small amounts of explosive material. Period bomb detection technology could not identify them. The only reliable method was physical search, which was impractical for large areas or moving targets. This undetectability made Time Pencils invaluable for clandestine operations.

Q: What is the Switch No. 9 “L-Delay” and how does it differ? A: The Switch No. 9 was developed by Millis Jefferis of MD1 as an alternative to chemical delays. Instead of using corrosive liquid, it employed a notched lead alloy wire under spring tension. The lead gradually stretched (mechanical creep) until it broke at the notch, releasing the striker. The L-Delay was more reliable underwater because it didn’t rely on chemical reactions that could be affected by water dilution. However, it generally offered shorter maximum delay times than the No. 10.

Q: Were Time Pencils ever used against Allied forces? A: Yes. German forces captured stocks of Time Pencils and used the technology for their own operations, including the July 20, 1944 assassination attempt against Hitler. The Germans also reverse-engineered the design and produced their own versions. Additionally, German stay-behind operations at the end of the war employed captured British demolition equipment including Time Pencils.

Q: How should encountered Time Pencils be handled today? A: Any suspected Time Pencil should be treated as extremely dangerous. Do not touch, move, or attempt to disarm it. Even devices over 80 years old may still be functional if the internal components remain intact. Immediately evacuate the area and contact military EOD or police bomb disposal units. Museum examples should have documentation confirming they have been properly rendered safe (detonators removed, mechanism disabled).

This lesson is intended for educational and training purposes. All ordnance should be considered dangerous until proven safe by qualified personnel. Unexploded ordnance should never be handled by untrained individuals—report findings to military or law enforcement authorities.