some bright spark left the papers next to the open window. The receiver in most systems also used two inductively coupled circuits, with the antenna an "open" resonant circuit coupled through an oscillation transformer to a "closed" resonant circuit containing the detector. The treatment was not painful, because currents in the radio frequency range do not cause the physiological reaction of electric shock. This system was necessary to give Marconi's transoceanic stations a narrow enough bandwidth that they didn't interfere with other transmitters on the narrow VLF band. [83][84][79][47], In addition to Tesla's system, inductively coupled radio systems were patented by Oliver Lodge in February 1898,[85][86] Karl Ferdinand Braun,[73][47][52][87] in November 1899, and John Stone Stone in February 1900. German physicist Heinrich Hertz in 1887 built the first experimental spark gap transmitters during his historic experiments to demonstrate the existence of electromagnetic waves predicted by James Clerk Maxwell in 1864, in which he discovered radio waves,[22] This is equivalent to a radio signal amplitude modulated with a steady frequency, so it could be demodulated in a radio receiver by a rectifying AM detector, such as the crystal detector or Fleming valve used during the wireless telegraphy era. Heinrich Hertz discovering radio waves with his spark oscillator (at rear), Hertz's drawing of one of his spark oscillators. One legacy of spark-gap transmitters is that radio operators were regularly nicknamed "Sparky" long after the devices ceased to be used. Their guidance was key, arranging training and probably providing weapons, and when the Serb government tried to get Apis to cancel he made little effort, leading to an armed group making the attempt in 1914. In powerful induction coil transmitters, instead of a vibrating interrupter, a mercury turbine interrupter was used. Each pulse of high voltage charged up the capacitor until the spark gap fired, resulting in one spark per pulse. Telefunken 25 kW long distance transmitter built 1906 at Nauen Transmitter Station, Nauen, Germany, showing large 360 Leyden jar 400 μF capacitor bank (rear) and vertical spark gaps (right), In developing these syntonic transmitters, researchers found it impossible to achieve low damping with a single resonant circuit. The efforts described above to reduce the damping of spark transmitters can be seen as attempts to make their output approach closer to the ideal of a continuous wave, but spark transmitters could not produce true continuous waves. At these wavelengths even the largest antennas were electrically short, a tiny fraction of a wavelength tall, and so had low radiation resistance (often below 1 ohm), so these transmitters required enormous wire umbrella and flattop antennas up to several miles long with large capacitive toploads, to achieve adequate efficiency. Nationalism was a prominent force in early 20th century Europe and a significant cause of World War I. [132][55] Because their energy is essentially concentrated at a single frequency, in addition to causing almost no interference to other transmitters on adjacent frequencies, continuous wave transmitters could transmit longer distances with a given output power. One of the first uses for spark-gap transmitters was on ships, to communicate with shore and broadcast a distress call if the ship was sinking. In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a switch called a telegraph key in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in Morse code. [47] Using a resonant circuit (also called tuned circuit or tank circuit) in transmitters would narrow the bandwidth of the radiated signal, it would occupy a smaller range of frequencies around its center frequency, so that the signals of transmitters "tuned" to transmit on different frequencies would no longer overlap. The energy in each spark, and thus the power output, was no longer limited by the capacitance of the antenna but by the size of the capacitor in the resonant circuit.