English: A "foxhole radio" receiver from World War 2. Not allowed powered radio receivers, which could give away their position to the enemy by the signal radiated by their
local oscillator, American soldiers in World War 2 constructed their own homemade
"crystal" radios so they could listen to news and music. The tuning coil
(cylinder) was attached to a long wire antenna and ground. For a detector, this used a graphite pencil lead attached to the point of a safety pin, pressing against a "blue" steel
razor blade. The graphite point touching the
semiconducting oxide coating of the razor blade formed a crude point contact
semiconductor diode which
rectified the radio signal from the coil, extracting the audio signal from the radio frequency
carrier wave, which was listened to with the earphones. The operator dragged the pencil lead across on the razor blade's surface until a sensitive spot was found and the station was heard, which required a lot of patience. The source says this foxhole radio belonged to Leutenant M. L. Rupert and was used on the Italian front.