Other major problems were that changing the operating frequency was a lengthy and complicated process, and unlike a spark transmitter, the carrier signal could not be switched on and off at will.
22.
Fessenden, experimenting with a high-frequency spark transmitter, successfully transmitted speech on December 23, 1900 over a distance of about, the " first audio radio transmission ".
23.
Radio experimentation at the University of Minnesota, initially using spark transmitters that could only transmit the dots-and-dashes of Morse code, began in 1912, conducted by Professor Franklin Springer.
24.
For a time Fessenden continued working with more sophisticated high-frequency spark transmitters, including versions that used compressed air, which began to take on some of the characteristics of arc-transmitters patented by Valdemar Poulsen.
25.
Smith was licensed on 3 July 1914 and one month later, he received a message from the Marconi spark transmitter at Poldhu in Cornwall, being sent to London, on the eve of the outbreak of war.
26.
After WWI, greatly improved transmitters based on vacuum tubes became available, which overcame these problems, and by the late 1920s the only spark transmitters still in regular operation were " legacy " installations on naval vessels.
27.
The Morse code signal of the spark transmitter consisted of pulses of radio waves called damped waves which repeated at an audio rate, so they were audible as a buzz or tone in a receiver's earphones.
28.
During World War I the original Marconi spark transmitter was replaced with an Alexanderson alternator, the invention of the General Electric engineer, with an output power of 200 kilowatts and looking like an ordinary power station generator.
29.
In 1904, Danish inventor Valdemar Poulsen had introduced the arc-transmitter, which, unlike the intermittent pulses produced by spark transmitters, created steady signals that could be used for amplitude modulated ( AM ) audio transmissions.
30.
During World War I, the original Marconi spark transmitter was replaced with an Alexanderson alternator, the invention of the famous General Electric engineer, with an output power of 200 kilowatts and looking like an ordinary power station generator.
