What Can We Learn from Replicating Hertz’s Electromagnetic-Wave Experiment?

Heinrich Hertz’s discovery of RF-band electromagnetic waves marked a milestone in electrical engineering. While we often attribute the origin of wireless technologies to Hertz’s experiment, his apparatus bore minimum resemblance to those familiar to us today. Connecting an induction coil to a dipole structure, he constructed a spark-gap discharger to generate and transmit pulsed oscillations. To detect them, he used a loop antenna and observed its induced sparks. Hertz’s discovery came in two phases. In 1887, he staged a setting to produce a signal pattern, which he believed to result from interference between electromagnetic waves propagating in air and along a wire. In 1888, he placed a metal plate opposite to the transmitter, and declared to obtain a standing-wave pattern due to transmitted waves and their reflections from the plate. Elsewhere one of us has shown and analyzed—through historical, experimental, and modeling studies—the crucial flaws in Hertz’s claimed discovery. In this paper, we further explore the pedagogical values of replicating Hertz’s experiment. At National Taiwan University, we rebuild Hertz’s spark-gap discharger and loop detector, and repeat his measuring procedures. To assist the measurements and unveil the physical characteristics of Hertz’s experimental system, we employ modern instruments such as an oscilloscope and a signal generator. We find that some parts of Hertz’s experiment or its modern variations have stable and clearly reproducible results. We can create a standing-wave pattern with his 1888 setup when the spark-gap discharger is replaced with a continuous-wave generator. We can also observe a standing-wave pattern, as he did, along a terminated wire with his 1887 setup, even when the energy source is a spark-gap discharger. Yet, some other parts—e.g., the standing-wave pattern with his 1888 setup using a spark-gap discharger—are difficult to replicate consistently. We believe that these different parts can serve as educational resources for different purposes. The more stable results can be demonstrated in class to instruct the basic principles of electromagnetic waves. The less stable results can be used in project-based teaching that provides opportunities for students to probe the complexity of a realistic experimental system, and to learn to cope with such a complexity.