the amount of energy originally stored in the capacitor. The radiated radio waves, along with the heat generated by the spark, uses up this energy, causing the oscillations to decrease quickly in amplitude to zero. When the oscillating electric current in the primary circuit has decreased to a point where it is insufficient to keep the air in the spark gap ionized, the spark stops, opening the resonant circuit, and stopping the oscillations. In a transmitter with two resonant circuits, the oscillations in the secondary circuit and antenna may continue some time after the spark has terminated. Then the transformer begins charging the capacitor again, and the whole cycle repeats. The cycle is very rapid, taking less than a millisecond. With each spark, this cycle produces a radio signal consisting of an oscillating sinusoidal wave that increases rapidly to a high amplitude and decreases exponentially to zero, called a damped wave. The frequency of the oscillations, which is the frequency of the emitted radio waves, is equal to the resonant frequency of the resonant circuit, determined by the capacitance of the capacitor and the inductance of the coil: The transmitter repeats this cycle rapidly, so the output is a repeating string of damped waves. This is equivalent to a radio signal amplitude modulated with a steady frequency, so it could be demodulated in a radio receiver by a rectifying AM detector, such as the crystal detector or Fleming valve used during the wireless telegraphy era. The frequency of repetition (spark rate) is in the audio range, typically 50 to 1000 sparks per second, so in a receiver's earphones the signal sounds like a steady tone, whine, or buzz. In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a switch called a telegraph key in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in Morse code. As long as the key is pressed the spark gap fires repetitively, creating a string of pulses of radio waves, so in a receiver the keypress sounds like a buzz; the entire Morse code message sounds like a sequence of buzzes separated by pauses. In low-power transmitters the key directly breaks the primary circuit of the supply transformer, while in high-power transmitters the key operates a heavy duty relay that