While the electric waves are radiated in all directions from the aerial, the _length_ of the waves set up is approximately four times the combined length of the aerial wires and the "lead in" connection to the oscillation transformer. The electric waves induce effective electrical oscillations in the aerial of the receiving station, even at distances of hundreds of miles, provided the receiving transformer, _RT_, is "tuned" in resonance with the transmitting apparatus by adjustments of the variable condenser, _VC_, and the loading coil, _L_. The _detector_ of these oscillations in the receiving transformer is simply a crystal of silicon or carborundum, _D_, in series with two telephone receivers, _Ph_. The crystal detector permits the electric oscillations to pass through it in one direction only. If the crystal did not possess this property, the telephone could not be used as a receiver as it cannot respond to high frequency oscillations. While one spark passes at _SG_, an intermittent current passes through the receiver in one direction. Since some 300 to 1200 sparks pass each second at _SG_ while the key, _K_, is closed, the operator at _Ph_ hears a musical note of this frequency as long as _K_ is depressed. Short and long tones then correspond to the dots and dashes of ordinary telegraphy. In order to maintain a _uniform tone a rotary spark gap_, as shown, is often used. This insures a tone of fixed pitch by making uniform the rate of producing sparks. The _Continental_ instead of the _Morse_ code of signals is generally employed in wireless telegraphy, since the former employs only _dots_ and _dashes_. The latter code employs, in addition to dots and dashes, _spaces_ which have sometimes caused confusion in receiving wireless messages. The United States government has adopted the regulations of the _International Radio Congress_ which directs that commercial companies shall use wave lengths between 300 and 600 or above 1600 meters. Amateurs may use wave lengths less than 200 meters and no others, while the government reserves the right to wave lengths of 600 to 1600 meters. See p. 459 for Continental telegraph code.