Circuit of an inductively coupled spark gap transmitter, an obsolete type of radio transmitter used from about 1900 to 1920. The inductively coupled transmitter, invented around 1898-1900 by Karl Ferdinand Braun and Guglielmo Marconi, was the first transmitter which had enough power for transoceanic communication, and the first that created sufficiently low interference that multiple transmitters could operate on different frequencies without interfering.    It became the dominant transmitter used during the wireless telegraphy era prior to 1920.
The advantage of the inductively coupled transmitter is that it had two tuned circuits: a "closed" tuned circuit L1, C1 and an "open" tuned circuit L2, C2 which radiated power through the antenna, tuned to the same resonant frequency.  These were "loosely coupled" together through their coils.  When the spark gap S discharged, it created oscillations in the circuit L1, C1.  This oscillating energy was gradually tranferred to the output circuit L2, C2, creating long "ringing" damped waves which had a narrower bandwidth than other spark transmitters, and thus less interference. 

The power comes from an induction coil T powered by a battery B.  The interrupter I breaks the primary current at a rate of 10 to 50 breaks per second, each break causing the coil to create a pulse of high voltage, 10-30 kV.   The primary coil L1 of the oscillation transformer forms a tuned circuit with the capacitor C1.  Each pulse charges the capacitor C1 until it reaches the breakdown voltage of the spark gap S.  C1 discharges through the spark gap and L1, creating radio frequency oscillating currents.  Coil L1 creates an oscillating magnetic field, which induces an oscillating voltage in the secondary coil L2.  C2 is not an actual capacitor, but represents the capacitance to ground of the wire antenna A. L2 and C2 together form a second tuned circuit which induces oscillating currents in the antenna, which radiates the energy as radio waves.  The transmitter's signal was a string of damped waves which repeated at an audio rate, and in a receiver sounded like a musical tone or buzz.   To transmit information with this signal, the operator tapped on the telegraph key K, turning the transmitter on and off, creating different length pulses of radio waves, to spell out text messages in Morse code.
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