Tremble! – The Answer to the Mystery Device

A couple of weeks ago I set blog readers a challenge regarding this device, who made it, what the mystery component in the bottom right corner was, and who rendered it safe.

Well done to KH for his (close, but not perfect) answer. I’ll be buying him lunch soon.  It’s actually a pretty interesting story.  The device was found placed next to a telephone junction box under a manhole cover in a street in Arthur Square, Belfast, Northern Ireland, laid by the IRA , in 1922.  The fuel can “Pratts Perfection Spirit” contained a home made mix and two improvised igniters (not a detonators) in parallel. The EOD team (about more of which shortly) recovered a wooden box with a single slider switch on the outside. Inside was the timing circuit, which had failed. There was a 4 volt “Ever-Ready” battery, an American made clock with a soldered wire switch connection.  The improvised igniter design is quite intricate with a thin copper wire running through magnesium flash powder held in a glass tube, but with a spark gap. The can contained about 20lbs of home made incendiary mix, based on sodium chlorate, some scrap metal and  handful of bullets.  When testing the explosive, it burnt with an intense heat, but interestingly also proved in some circumstances to be “detonable”. I’m leaving out details of the mix for obvious reasons.

Here’s a circuit diagram , done in follow up investigation and analysis.

(This device did not work)

The mystery component is, I think, very interesting, but received scant attention at the time. It is described as a “trembler” but it would be wrong to think of it as an anti-handling trembler switch.  It is in fact an induction coil device for upping the voltage from the 4v Ever-Ready to sufficient voltage to cause the igniters to act as designed. It is actually a car component from a Ford Model T.  This component was known as a “trembler” or “buzzer coil”, and provided sufficient voltage for a car’s ignition system (several thousand volts).  Here’s a video explaining this component.

These trembler switches were popular with ham radio enthusiasts and early electronics hobbyists as an easily available and reliable component.

Finally we come to who rendered it safe. For many recent decades, the lead military agency for EOD in Northern ireland was the Royal Army Ordnance Corps who morphed in the 1990s to the Royal Logistic Corps. And very proud of it we were too!  But in the 1920s, it was the Royal Engineers who provided their expertise to deal with the device and many others.  I can hear my spiritual foundations shaking…

Other devices dealt by the Sappers near Armagh that year were cast into concrete to look like kerb-stones, a technique used more recently in Iraq. They were initiated by command-wire.

Update: Render Safe Procedure used in 1922

I have been asked about the Render Safe Procedure (RSP) used by the Royal Engineer EOD personnel  on this device in 1922. I don’t often discuss these things for obvious reasons but I think I’m OK with this one and its quite interesting.  Here’s what they did:

1. Filled the manhole with water from a fire hose, submerging the device. Gave it a three hour “soak”.
2. Removed the wooden box (which was in a sack) from the manhole, cut open the sack. There was a concern over a possible booby trap switch attached to the sacking but none was
3. The external slider switch was cut off manually, leaving an open circuit
4. The wooden box was pried open, and components separated after photographing
5. The cap of the fuel can was removed manually  and the “sand like” HME observed, with the leads leading in.
6. There was a small 1″ diameter hole in the base of the can covered with some sort of cover.  Apparently this concerned the operator as it may have indicated something clever included in the devices construction. Rather than pull the leads out through the cap, or open the tin with a tin opener or hammer and chisel, the explosive was carefully removed, through the bottom 1″ hole, bit by bit with a a long gouge to eventually reveal the igniters (at this stage assumed to be detonators/blasting caps). These were then cut out.
7. A series of tests were conducted on the components, quite thoroughly.