File:EB1911 Telegraph - Marconi transmitter and receiver.jpg

English: Guglielmo Marconi's early wireless telegraphy transmitter (left) and receiver (right) from the 1890s with which he performed the first radio communication experiments.

The spark gap transmitter consisted of an induction coil (I) powered by a battery (B) connected to a spark gap (S) consisting of two metal balls with a gap of a few millimeters between them. One spark ball was connected to a monopole antenna (A₁) consisting of a wire that went up to a metal sheet suspended high overhead. The other spark ball was connected to ground (E). High voltage from the induction coil charged up the antenna to a high voltage which was discharged by a spark between the spark balls. The spark excited oscillating standing waves of current in the antenna; the charge flowed back and forth between the ground and the antenna through the spark, charging the antenna alternately positive and negative. The antenna radiated this energy as radio waves. To communicate information with this device the operator tapped on the telegraph key (K) in the primary circuit of the induction coil, turning the transmitter on and off rapidly, transmitting pulses of radio waves which spelled out text messages in Morse code.

The receiver (right) consisted of a similar metal plate monopole antenna (A₂) connected to a primitive radio wave detector called a coherer (C), consisting of a glass tube with electrodes at either end with loose metal powder between them. Radio waves from the transmitter created an oscillating voltage in the receiving antenna which was applied to the coherer. The coherer was also connected to a DC circuit with a battery (B₁) and relay (R). The radio voltage caused the metal powder to "cohere" or stick together, which lowered the resistance of the coherer. Current from the battery flowed through the coherer and actuated the relay, applying current from the second battery (B₂) to a Morse siphon recorder (M) which recorded the pulses of the messages as ink lines on a paper tape. The receiving operator later read the tape, translating the Morse code pulses back into text, writing down the message.