Forgotten audio formats: Wire recording

From espionage to home recording, the colourful life of the longest-used audio medium.

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[Mazarin]

But later on, during the 1930s and again in World War II, the secret services of several nations would use wire recordings, massively sped up, to broadcast on shortwave radio to their agents in the field. The agents would be standing by, ready to "wire tape" these broadcasts, already aware of the exact tempo to play them back at. Of course, the enemy could sometimes decipher these broadcasts if they had enough experts and wire recorders with which to try speed experiments...

That's silly. The reason "squirt" transmissions were used was to make them more difficult to intercept and record by the enemy, and to prevent triangulation of the sender. The latter was particularly important if the sender was a field agent in an occupied country. If the transmission was recorded by the enemy, then slowing it down so that the characters could be distinguished was a trivial problem. Usually, the messages would be sent in Morse or a similar encoding scheme, so you would just slow the recording until you got the dots and dashes. After that, reading the message was a question of how well it was encrypted in the first place.