The inventions by Heinrich Hertz of the first wireless radio transmitter and receiver were the result of a process of trial and error, followed by systematic experiments to confirm his results. The primary of his induction coil was fitted with an electromechanical vibrator to interrupt the current and produce a pulsed high voltage across the secondary. His antenna oscillator was a dipole consisting of two zinc-covered spheres acting as charge-storing capacitors. Each sphere was connected with a variable length of wire to the terminals of a spark micrometer, which in turn were connected to the secondary of the induction coil. The separation of the spheres could be varied up to 3 meters to support half-wavelength oscillations at resonance, thus producing up to 6-meter wavelengths that were short enough for convenient indoor measurements. The high-voltage pulses charged the spheres until a spark discharged them. About five or six discharges in each pulse produced oscillating currents in the antenna up to 50 megahertz.

At first, Hertz used a loop of wire connected to another spark micrometer as a way of measuring the resonant frequency of the antenna oscillator. When the loop radius was adjusted to resonance, it sparked at the same time as the oscillator. He could then determine their common frequency by calculating the loop frequency. When he found that he could obtain synchronous sparks several meters away, he realized that his antenna oscillator was a wireless transmitter and that he was detecting electromagnetic waves from the transmitter with his variable loop, which was the first radio receiver. He was even able to detect these sparks in an adjacent hallway as the radio waves passed through the walls.

After these inventions, he began a series of experiments in the fall of 1887 to demonstrate that the radiation from his transmitting oscillator corresponded to James Clerk Maxwell's prediction of electromagnetic waves traveling at the finite speed of light. He set up his induction coils and oscillator at one end of a 15-meter lecture hall and covered the opposite wall with a sheet of zinc to reflect the radiation. When he moved his receiving loop and spark gap away from the oscillator, he observed a periodic variation in the strength of sparks, with nodes about 4.8 meters apart, corresponding to a 9.6-meter wavelength in the standing waves produced by the transmitted waves and their reflection. This value was distorted to some extent by the small size of the room relative to the wavelength.

With the radius of the loop at about 35 centimeters, Hertz calculated a resonant frequency of 35.7 million vibrations per second (megahertz), giving a wave speed (frequency x wavelength) of 3.4 x 10 meters per second, about 13 percent more than the measured speed of light. Later corrections by Henri Poincare showed that the frequency was 50 megahertz and the wavelength was 6 meters, giving a speed of 3 x 108 meters per second, matching the speed of light. These experiments were completed in March of 1888 and published in his paper "On Electromagnetic Waves in Air and Their Reflection," initiating a new era of wireless communication.

The main impact of Hertz's work came from his invention of radio and the associated discoveries of radio waves and the photoelectric effect. Although Hertz was primarily interested in clarifying electromagnetic theory, and never mentioned applications of his work, radio opened up new communication technologies and new extensions of the electromagnetic spectrum. One of the earliest applications of radio waves was by the Italian engineer Guglielmo Marconi after reading papers by Hertz in 1894. Marconi replaced the spark gap between the ends of the dipole antenna with a tube of metal filings called a coherer, which greatly increased the current in the antenna. By 1896, he had succeeded in sending coded messages far enough to warrant a patent, and by 1898 he had transmitted signals from Ireland to Scotland. On December 12, 1901, he sent wireless signals across the Atlantic from England to Newfoundland, using balloons to lift his antennas as high as possible.

The work of Hertz clarified and confirmed electromagnetic theory, leading to both relativity and quantum theory. It opened up a new understanding of the entire spectrum of electromagnetic waves from radio to X-rays, all of which travel at the speed of light but differ in frequencies and wavelengths. The absolute value of the speed of light was the basis for Albert Einstein's special theory of relativity, proposed in 1905. The photoelectric effect was studied systematically after Hertz died by his assistant Philipp Lenard, who showed in 1902 that the energy of photoelectrons increases with the frequency of the incident light that produces them. This discovery was used by Einstein in 1905 to confirm and generalize quantum theory, and it is the basis for such devices as digital cameras, in which light signals are changed into electric currents.

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