The transmitter was powered by a 40 hp steam engine turning a 125 Hz 35 kVA alternator (AC generator) (in foreground) whose output current was stepped up by a transformer to thousands of volts. This powered a tuned circuit consisting of a capacitor and air core coil through the rotary spark gap (visible), consisting of a wheel with 50 electrodes on its rim which passed by a stationary electrode. The sparks excited oscillating radio frequency currents in the tuned circuit which were applied to the antenna through a secondary coil coupled to the first.

Fessenden's machine had two advantages:

• The high separation speed of the electrodes on the wheel quenched the spark early in the electrical oscillations, allowing the secondary circuit consisting of the antenna and secondary coil to oscillate freely. This created long "ringing" waves. Earlier spark-gap transmitters had allowed the spark to continue throughout each output wave, dissipating the energy from the capacitor in the heat of the spark.
• The rotary spark wheel was "synchronous". The spark wheel was turned by the alternator shaft and thus the sparks were synchronized with the AC sine wave from the generator, occurring on the peaks of the sine wave. This created a 750 Hz note with lots of harmonics that sounded "musical" in the receiver headphones. This musical signal cut through atmospheric static better than previous "asynchronous" spark wheel transmitters, whose signal sounded like an unmusical "buzz" in the receiver.