English: w:Crystal radio receiver with headphones probably from around 1920. Offered for sale at the MIT Flea Market.

This was a circuit commonly used in homemade crystal radios in the 1910s and 1920s, called a "two-slider" circuit. The radio was used with a long wire antenna, and the capacitance of the antenna resonated with the tuning coil to tune in stations, so no tuning capacitor is used. The tuning coil (coil of wire) has two adjustable contacts which could be slid along the coil, to include a greater or lesser number of turns in the circuit. One was connected to the antenna, and the other was connected to the galena crystal detector (mounted next to the coil) and earphones in series.

In a crystal radio, all the power comes from the antenna, so it is important to transfer as much power from the antenna to the earphones as possible. Maximum power is transferred from one circuit to another when the two circuits are impedance matched, with the source impedance of the first equal to the load impedance of the other. However in a crystal radio, the resistance of the antenna/ground circuit, about 10 - 200 ohms, is much lower than the tuning coil (several thousand ohms at resonance), and varies with the antenna length, grounding and frequency. The coil resistance is also usually higher than the crystal and earphones circuit. So the antenna and the earphone circuits are connected across only a portion of the coils turns with the adjustable contacts. The coil, in addition to forming the tuning circuit for the radio, also serves as an impedance matching transformer, scaling up the antenna impedance to match the tuned circuit, and scaling the earphone impedance to match the coil. As the radio is tuned to different stations by moving the antenna contact, the other contact is adjusted until the station sounds loudest in the earphone.