English: Marconi type 103 crystal radio receiver, used with the Marconi company's spark transmitter systems during the pioneering wireless telegraphy era of radio around 1920. The cat's whisker detector is visible at top. This radio used an inductively-coupled antenna transformer circuit, which improved the selectivity (Q factor) of the receiver, allowing interfering stations near the desired station to be rejected. The "short wave condenser" tuned the primary coil, the "intermediate condenser" tuned the secondary. The "aerial tuning inductance" knob selected taps on the coil winding to adjust the impedance match between the antenna and the receiver. The "Billi condenser" was a decoupling capacitor across the earphone leads, which smoothed the audio signal from the detector, removing the residual RF carrier pulses. The "potentiometer" applied an adjustable bias voltage from a battery across the detector, to make it more sensitive.