German physicist H. Hertz (1857 ~ 1894), although he only lived for 37 years, made two major discoveries: first, he confirmed Maxwell's predicted electromagnetic waves in experiments; The second is the discovery of photoelectric effect.

In 1970s, when Hertz started his scientific activities, people's understanding of electromagnetic phenomena was still in a state of inconsistency. Maxwell's electromagnetic theory has just been put forward. Because this theory uses advanced and novel mathematical tools, and because the concept of Newtonian mechanics has been deeply rooted in people's hearts and the macroscopic mechanical phenomenon is intuitive, it has not been generally accepted. Many physicists are still confined to the framework of mechanical theory, trying to establish electromagnetic theory according to the framework of mechanical theory. The key of Maxwell's theory is displacement current and electromagnetic wave. The existence of electromagnetic waves is predicted theoretically, and it is proposed that light is a kind of electromagnetic waves. Electromagnetic waves should have a wide frequency range, and the frequency range of light waves only accounts for a small part. To prove the correctness of Maxwell's theory, it is necessary to prove the existence of electromagnetic waves with other frequencies through experiments. It also travels at the speed of light and has the characteristics of reflection, refraction, diffraction, interference and polarization like light waves. So 1879, the Prussian Academy of Sciences in Berlin offered a reward for the experimental verification of electromagnetic waves.

Hertz was a student of Helmholtz, and Helmholtz admired him very much. Teachers and students have maintained close friendship all their lives. Helmholtz called it the field of electromagnetism at that time? Quot wilderness without roads ",he set himself the task of conducting a comprehensive study in this field, trying to sort out this chaotic state; In fact, it was Helmholtz who drew up the award of the Berlin Academy of Sciences. Influenced by it, Hertz deeply studied the electromagnetic theory. He is determined to carry out the experiment of rewarding answers by the Academy of Sciences. However, due to other work reasons, this matter has been put on hold for several years.

The experiment of Hertz confirming the existence of electromagnetic waves was completed in 1887 ~ 1888. The electromagnetic wave generator and detector he used. On the left is the generator, which is made up of two small copper balls that are very close to each other and connected with a big copper ball through a 30 cm long copper rod. Two large copper balls are equivalent to two plates of a capacitor, with a capacitor in the middle and an inductor in the copper bar. Connect the output of the induction coil to two small copper balls to charge the capacitor. When a certain voltage is reached, a spark short circuit occurs between two small copper balls, and the generator becomes an LC loop, and the charge on the capacitor is discharged by spark, resulting in high-frequency oscillation (because the inductance and capacitance of the loop are very small). Due to the shape of the capacitor, the electric field permeates the whole space, producing electromagnetic waves that propagate outward. On the right is the detector, which is formed by bending a copper wire into a circle (the radius adopted by Hertz is 35 cm) and welding two copper balls at both ends. The distance between the two balls can be adjusted. It is also an oscillating circuit. The capacitance between two balls is the capacitance of the circuit, and the natural frequency of the circuit is determined by its inductance and capacitance. In order to achieve significant detection effect, the detector is tuned to resonate with the generator. In this way, when the electromagnetic wave arrives, the electromotive force is induced on the circular copper wire of the detector, and forced oscillation is generated in the loop. Due to resonance, the loop in the detector produces strong oscillation. At this time, sparks will appear in the spark gap, so the existence of electromagnetic waves can be tested. Hertz also measured the wavelength of electromagnetic wave as 66 cm by moving the detector to different positions, which is 106 times of the wavelength of light wave. According to the wavelength and the calculated oscillation frequency, it can be calculated that the wave velocity is equal to the speed of light.

Later, Hertz also realized the reflection of waves and verified the law of reflection. The superposition of the original wave and the reflected wave produces a standing wave, which confirms the interference. Hertz also refracts electromagnetic waves through asphalt prisms; Diffraction can be observed through a perforated shield; Parallel wire grids generate polarization; Cylindrical metal screens are also used to focus electromagnetic waves. These experimental results show that the properties of electromagnetic waves are the same as those of light waves. In this way, Hertz proved the correctness of Maxwell's theory by experiments, and the electromagnetic theory began to be accepted by many scientists. By the end of 19, Maxwell's theory had occupied a dominant position in the field of electromagnetism.

Hertz also found the photoelectric effect by the way in the electromagnetic wave experiment. In 1887, he found that when the two poles of the detector oscillator are illuminated by the spark light of the transmitter oscillator, the spark of the detector will be strengthened. Further research shows that this is due to ultraviolet radiation, which will knock negatively charged particles out of the negative electrode. He published the matter as a paper, but did not study it further.

1894, Hertz died of toxemia caused by dental diseases before he was 37 years old. In memory of Hertz, his name was used as the name of the frequency unit.

Hertz is not only an excellent experimental physicist, but also has a good theoretical literacy. 1884, he introduced the vector potential a into the electromagnetic theory. 1890, he rewritten Maxwell's equations from the original form (***8 equations, including 6 vector equations) into a simplified symmetrical form, including only 4 vector equations, which is still in use today. His system is concise and vivid, which accelerates the spread of Maxwell's theory. He also wrote a book, Principles of Mechanics (Expressed in a New Form), which was published behind him. This book not only restates the achievements of predecessors, but also includes some new ideas of its own.

Although Hertz studied engineering when he was young and was a professor at the engineering university when he did electromagnetic wave experiments, he pursued the understanding of the basic laws of nature and did not care about the practical application of electromagnetic waves. After discovering electromagnetic waves, he turned to Maxwell's theory and basic principles of mechanics. In addition, he died young, so Hertz himself did not consider the possibility of transmitting information by electromagnetic waves. However, the gap has widened and the conditions are ripe. Hertz set the stage for Marconi and popov, and the invention of radio was a historical necessity. Many people devoted themselves to the research on the application of electromagnetic waves, and achieved concrete results within one or two years after Hertz's death. Radio electronics developed rapidly throughout the 20th century, creating today's information age.