Literal underground communication

Sixth and last in a vaguely defined series on obscure spy tech, after SCR-268 and Tenzor and Peilempfänger and Kleinstpeilempfänger and Keinpeilempfänger.

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Encryption and secrecy are the natural default for language. We communicate with our family or tribe or army, and we DO NOT WANT other families or tribes or armies to understand our meaning.

Electric telegraphy started with the same purpose. Visual semaphores, whether by hand or by machine, were visible to anyone nearby. A watcher might not know the code, but he could determine who was talking and who was listening, which is 80% of the interceptor’s job.

The earliest attempts at sending a signal by electricity were underground or underwater. Electricity promised a return to natural secrecy. This purpose was forgotten as telegraphy became widespread and commercial. Telegraphy moved aboveground in wires, which turned out to be ‘secret enough’ for most purposes.

Telegraphy through the ground continued in special situations like mines. In WW1 the French succeeded in using regular spark-gap wireless transmitters and receivers for tunable underground communication. The US Signal Corps picked up the idea and the name.

Telegraphie Par Sol = TPS.

This French transceiver caught my eye mainly because of the beautiful tubes. Round iodine-colored long-legged tubes were typical of French radios in the 1910’s. (Tubes, unlike other internal components, had distinct national styles.)**

It’s not well documented, so I’m partly guessing. The Morse key at left is obvious. Lower left is the induction coil with breaker points. The DPDT switch at upper right would have been transmit/receive. The rotary switch might have tuned the receiver, which was also purely audio. No detector or demodulator, just three pretty stages of audio onstage.. The four SPST switches at top center have unknown purposes.

The breaker points were adjustable for various frequencies, all in the audio range. There was no output in the usual radio range; the secondary of the coil simply imposed an alternating high voltage across the two sets of ground spikes. The typical freq was 200 cps, G below middle C.

Polistra is sending her usual message to the gods, and HappyStar is indicating the slow alternation of the electric field between the two sets of ground spikes. (Well, not quite that slow, but 200 cycles is glacial compared to even VLF radio freqs.)

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Seen from the top, the waves would gradually spread out through the surface of the soil.

The American version has a good tech manual online, with advice for choosing locations and placing the sending and receiving rigs.

The best soil for this purpose was also the best farmland. Dry or rocky soil doesn’t conduct at all. Swamp (dirty water) has very low resistance, so the current will simply flow between the rods and won’t try to spread out. In good agricultural soil the current takes a much broader path between and around the sets of spikes, so the field has a chance of reaching a location within a mile or so.

Plants use electricity to communicate underground through fungal wires, and plants use electric fields to talk with bees. Plants seem to have the same preferences as TPS. Coincidence?

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** Sidenote on national tubes: British tubes were small and cylindrical. The 1920s Brit tubes look more like the typical ‘miniature’ tubes of the 1950s, and the Brits started using subminis before we did. The first American tubes looked like baseball bats. The more familiar American style was the Coke bottle. Hungarian tubes were squat Coke bottles with totally unique sockets. As far as I can tell, Italy and Germany didn’t have distinctive tubes, though they did have recognizable cabinets and cases.

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For Poserites: Cinched up the set and released it to ShareCG.