Metalloid silicon, germanium, arsenic, antimony, and tellurium are commonly recognised as metalloids. Depending on the author, one or more from selenium, polonium, or astatine are sometimes added to the list. Boron sometimes is excluded, by itself, or with silicon. Sometimes tellurium is not regarded as a metalloid. The inclusion of antimony, polonium, and astatine as metalloids has been questioned. Other elements are occasionally classified as metalloids. These elements include hydrogen, beryllium, nitrogen, phosphorus, sulfur, zinc, gallium, tin, iodine, lead, bismuth, and radon. The term metalloid has also been used for elements that exhibit metallic lustre and electrical conductivity, and that are
Crystal detector voltage of around 0.2V from a battery to make it more sensitive. Although the zincite-chalcopyrite "Perikon" was the most widely used crystal-to-crystal detector, other crystal pairs were also used. Zincite was also used with carbon, galena, and tellurium. Silicon was used with arsenic, antimony and tellurium crystals. During the first three decades of radio, from 1888 to 1918, called the "wireless telegraphy" or "spark" era, primitive radio transmitters called spark gap transmitters were used, which generated radio waves by an electric spark. These transmitters were unable to produce the continuous sinusoidal waves which are used to transmit audio (sound) in