and tellurium. Silicon was used with arsenic, antimony and tellurium crystals. History 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 modern AM or FM radio transmission. Instead spark gap transmitters transmitted information by wireless telegraphy; the user turned the transmitter on and off rapidly by tapping on a telegraph key, producing pulses of radio waves which spelled out text messages in Morse code. Therefore, the radio receivers of this era did not have to demodulate the radio wave, extract an audio signal from it as modern receivers do, they merely had to detect the presence or absence of the radio waves, to make a sound in the earphone when the radio wave was present to represent the "dots" and "dashes" of Morse code. The device which did this was called a detector. The crystal detector was the most successful of many detector devices invented during this era. The crystal detector evolved from an earlier device, the first primitive radio wave detector, called a coherer, developed in 1890 by Édouard Branly and used in the first radio receivers in 1894–96 by Marconi and Oliver Lodge. Made in many forms, the coherer consisted of a high resistance electrical contact, composed of conductors touching with a thin resistive surface film, usually oxidation, between them. Radio waves changed the resistance of the contact, causing it to conduct a DC current. The most common form consisted of a glass tube with electrodes at each end, containing loose metal filings in contact with the electrodes. Before a radio wave was applied, this device had a high electrical resistance, in the megohm range. When a radio wave from the antenna was applied across the electrodes it caused the filings to "cohere" or clump together and the coherer's resistance fell, causing a DC current from a battery to pass through it, which rang a bell or produced a mark on a paper tape representing the "dots" and "dashes" of Morse code. Most coherers had to be tapped mechanically between each pulse of radio waves to return them to a nonconductive state. The coherer was a very poor detector, motivating much research to find