English: The current in the primary (I₁) and secondary (I₂) circuit of a inductively coupled spark-gap transmitter, an obsolete type of radio transmitter. The circuit consists of an air core resonant transformer whose primary is connected in a circuit with a capacitor through a spark gap, forming a tuned circuit and whose secondary is connected to an antenna and ground. The secondary also forms a tuned circuit with the capacitance of the antenna. The two circuits are tuned to the same resonant frequency. The capacitor is charged with high voltage by a supply transformer. When the capacitor is discharged through the spark gap, the energy creates oscillating radio frequency current in the primary winding. The current creates a magnetic field which induces current in the secondary winding. The energy is transferred to the secondary winding, and the oscillating current in the primary circuit decreases to zero while the current in the secondary increases to a maximum. Then the energy is transferred back to the primary circuit, the secondary current declining and the primary current increasing. This process repeats, with the energy transferring back and forth between primary and secondary, as the amplitude of the current gradually declines due to energy radiated away by radio waves from the antenna. This oscillation wastes energy in the spark, and causes the transmitter to radiate on two separate frequencies.

In a quenched spark transmitter, the spark in the primary circuit is extinguished (quenched) at nodal point Q when the primary current momentarily goes to zero after all the energy is transferred to the secondary. This uncouples the two circuits, allowing the secondary to oscillate independent of the primary after that, creating long ringing waves that have low damping, and consequently a very pure, small bandwidth radio signal. This is shown in the companion diagram Spark-gap transmitter current waveforms - quenched gap.png.