English: A precision crystal detector (sometimes called a cat's whisker detector) from around 1914, an obsolete radio component used in unamplified crystal radio receivers. This type was a professional unit used in sensitive receivers in wireless telegraphy stations. The vertical needle on the right gently touches the face of a crystal of iron pyrite (iron sulfide, FeS₂, Fool's gold) contained in the cylindrical metal capsule below it, forming a diode which is used to rectify the received radio signal. Only certain sites on the crystal's face functioned as rectifying junctions, so the device had to be adjusted by trial and error to find a "sweet spot" before each use. The crude device was very sensitive to the contact between needle and crystal, so the spring loaded micrometer screw on the left can be used to adjust the pressure precisely.