The gentleman shown above is demonstrating a simple wireless telegraph, the plans for which appeared 75 years ago this month in the December 1940 issue of Popular Science. As you can see from the diagram here, the concept was nothing new, and is quite simple.  The transmitter sends an AF signal of about 75 volts at the buzzer’s frequency into two ground rods.  The receiver is also hooked up to two ground rods, and consists of an audio amplifier to detect the weak audio signal being sent into the ground.  This set was said to have a range of about 200 feet.

The concept is similar to the WW1-era field buzzer which appeared here previously.  With even a very weak audio signal, it’s easy to pick out the dots and dashes of Morse code, even though the signal is far too weak to make out speech.

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