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The P-4 detector radio has been producing the Moscow Telegraph Plant (former Morse plant) since 1926. The radio is designed to receive radio stations in the range from 300 to 1800 meters. It is assembled in a wooden box, pasted over with dermantine, dimensions 160x78x115 mm. The basis of the radio scheme is a coil, which is wound on a wooden blank of an elliptic form. 180 turns of enamelled copper wire with a diameter of 0.5 mm are wound on a blank. The inductance of the coil is changed by rotating the knob of the rheostatic slider, the contact of which moves along the turns.