Heinrich Hertz’s spark experiment. How electromagnetic waves travel through space.
Capacitor, dielectric, conduction current, displacement current and flow of electrons:
• Capacitor, dielectric, conduction current,...
James Clerk Maxwell had already predicted that light itself was an electromagnetic wave — but no one had ever seen electromagnetic waves being produced or detected directly. That’s where Heinrich Hertz comes in. His goal was simple but revolutionary: to prove that electromagnetic waves could be generated, transmitted through space, and detected — all without any wires. Here’s the setup Hertz used in his experiment. At the center of the system, he used something called a Ruhmkorff induction coil, which was basically a high-voltage transformer. On the left side of the coil — the primary winding — he connected a simple DC battery and an interrupter. The interrupter repeatedly opened and closed the circuit, causing the magnetic field in the coil to build up and collapse rapidly. Every time the magnetic field collapsed, it induced a very large voltage pulse in the secondary coil, on the right side — sometimes tens of thousands of volts. Those two high-voltage outputs were connected to two large metal plates, which I’m drawing here. When the coil fires, one plate becomes positively charged and the other becomes negatively charged. The air between them acts as an insulator, so the charges can’t recombine immediately. That separation of charge creates an electric field, storing energy between the plates — just like a capacitor does. As the voltage keeps increasing, the electric field between the plates becomes so strong that it ionizes the air, and a spark suddenly jumps across the gap. Now, that spark is the key moment. When it fires, the stored energy in the plates is released, and it flows through the connecting wires and the coil. Together, the inductance of the coil and the capacitance of the plates form a resonant LC circuit — meaning the energy now oscillates rapidly between the electric field in the plates and the magnetic field in the coil. These oscillations — millions of times per second — generate electromagnetic waves, which radiate outward through space. So, what started as a spark becomes a self-sustaining electromagnetic oscillation — the first ever man-made radio wave. Now, a few feet away, Hertz placed a receiver loop, which I’ll draw here. It’s just a circular loop of wire with a tiny gap in it. When the electromagnetic wave from the transmitter passes through, it induces a small alternating current in the loop — and if the loop is tuned to the same resonant frequency, a tiny spark appears across the receiver’s gap. That spark is visual proof that electromagnetic energy has traveled through the air — wirelessly. This simple but brilliant setup confirmed everything Maxwell had predicted. It showed that changing electric and magnetic fields can sustain each other and propagate through space as waves — the same kind of waves as visible light, just at much lower frequencies. And that’s how Hertz, using nothing but sparks, coils, and metal plates, became the first person to generate and detect electromagnetic waves — the birth of modern wireless communication. So next time you see a radio antenna or connect to Wi-Fi, remember — it all started right here, with Hertz’s spark gap and a couple of capacitor plates lighting up the air with invisible waves.