English: The huge tuning coil (oscillation transformer) of the 300 kW transatlantic spark transmitter built in 1916 by the Marconi Wireless Telegraph Co at Carnarvon, Wales. This was the most powerful spark transmitter ever built, transmitting commercial radiotelegraphy traffic on a frequency of 21.5 kHz at a rate of 200 words per minute to receivers in Tuckerton, New Jersey and the RCA Radio Central station, Long Island, New York. The 5 ft diameter primary winding of the oscillation transformer is in center, consisting of 3 turns of special litz wire one foot in diameter, consisting of many strands of insulated wire around a jute core. It was connected to a huge capacitor bank and rotary spark gaps which filled the room below. This huge conductor was needed to reduce resistance from skin effect, reducing the damping of the tuned circuit to keep the bandwidth of the powerful signal radiated by the transmitter narrow, to prevent it interfering with other transmitters on nearby frequencies. The 6 turn secondary winding which feeds the antenna is located on the same axis, hidden behind it. The other coils visible are antenna loading coils. The transmitter was obsolete by the early 1920s, superseded by vacuum tube transmitters.