CRYSTAL RADIO

Crystal radio sets became available in the early 1920s. A friend kindly gave me a Bijou model manufactured by the British Thomson Houston company (BTH).

Several companies also started selling kits to build your own crystal radio. I have acquired such a nameless specimen. The tuning is not very accurate where the LC circuit is defined by the antenna capacitance and a variometer coil. The latter is a combination of a fixed outer coil and a rotating inner coil, both connected in series. Audio detection is by finding a sweet spot on a galena (PbS) crystal using a flexible wire (cat's whisker). The signal is just about strong enough for a set of high impedance earphones.