In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current the antenna radiates radio waves. In addition to their use in broadcasting, transmitters are important components in electronic devices that communicate by radio. These come in the form of mobile phones, radio devices, even garage door openers are operated by radio frequencies. The term transmitter is normally limited to equipment that generates radio waves for communication purposes, but such devices as microwave ovens and diathermy equipment use the same technology and quite often use similar circuits. The term is popularly used more specifically to refer to transmitting equipment used in broadcasting as in radio transmitter and television transmitter. The very first primitive radio transmitters were built by the German physicist Heinrich Hertz in 1887, these early transmitters generated radio waves by a high voltage spark between two conductors. These spark gap transmitters were used during the first three decades of radio from 1887 to 1917; this was known as the wireless telegraphy era.