Wireless of the Week - week 15

[Bruce Parker (RIP)]

At the beginning of the First World War the British began wondering who the spy was. Every time they planned something the Hun (I say ‘Hun’ with all due respect to our German friends) was there waiting for them. It was soon clear, however, that the ‘spy’ was the lowly signaler communicating his signals from trench to trench over his field phone and, through no fault on his part, offering up the same message to the Germans by way of their sensitive interception equipment.

Field phones operating with morse code send their dots and dashes as electric pulses over wires: current for a dot or dash, no current for the space between them. It was this electric on/off that was being picked up. Captain Fuller, a British Signals officer came up with a device to beat the Germans. What if you sent a signal for your dot and dash, but then put a similar current on the line for the silent space between them? To another sensitive Fullerphone, the distinction was heard and the signal received. The eavesdropper got a continuous current in the line and therefore had no idea what was being communicated. Further frustrating the Hun (again apologies, but it does fit the narrative) was the very low amperage required for signals between sets.

The continuous current in the line while still sending a morse signal was accomplished with a ‘buzzer chopper’ that sent a current over the line while also providing current to a series of condensers. When the circuit was broken, the condensers released their current into the line at much the same voltage as that of the morse dot or dash.

Beyond the clandestine use of Fullerphones, the different nature of their signals also allowed them to operate on the same line as a regular field phone without any interference with the field phone operation. This meant one line could be used for two or more communication devices saving effort, time, material…and lives. Range of a Fullerphone is only limited by leakage and resistance in the line however under field service conditions the range called for would not normally exceed 15 to 20 miles.

This Fullerphone is a Canadian made Mk.IV example built by Northern Electric in 1941. It is stored in a wooden box 12” by 6-7/8” by 7” with a web carrying strap and provision for a grounding spike. The complete unit weighs 14 pounds. A set of earphones is held on the lid by metal clips when not in use and the lid also has a metal plate with operating instructions. The set is removed from the box by depressing a metal tab and sliding it forward. It is a metal alloy case that has provision for two X cell batteries, terminal posts for the line connections, a potentiometer dial (to reduce interference from stray earth voltages), plugs for the headset and a built in morse key. The ‘buzzer chopper’ is a removable component on the top right and has controls with locks to the set buzzer and tone in the earphones.

The manual has several diagrams showing how Fullerphones could be used in pairs or in complex multiple configurations alongside regular field phones and connected to exchange switchboards.

Hopefully now when someone calls a field phone a Fullerphone you can set them straight.