I have a question please about sparks used in the early history of radio. I have read the similar questions but I still need your help please.

I have learned from sources such as https://neurophysics.ucsd.edu/courses/physics_1b/SerwayCP7_Ch21.pdf and http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf that AC, whether in wire or spark, creates EM waves. This is because in AC the electrons are constantly accelerating and decelerating, and acceleration of a charged particle is required to generate EM waves.

I am using 2 HV generators to generate sparks and a neon bulb to test for presence of EM waves.

(1) The first generator is a small enclosed plastic cylinder. I give it 5VDC input with low amps. I couldn't find specs for output except it claims to generate 400KV output which I doubt. This generator continually produces an intermittent spark which causes the neon bulb to flash in sync.

(2) The second generator is a ZVS flyback transformer ("12v-36V Zero Voltage Switching(ZVS) Tesla Coil Flyback Driver Circuit"). I give it 24VDC at 10A. This generator continuously produces a solid spark which causes the neon bulb to stay lit continuously. The specs for this generator state that it generates "High voltage direct current output, voltage input voltage of about 1000 times."
If this is true then it seems to be that the DC spark is creating EM waves, which does not make sense to me. How is the neon bulb lighting up?

  • 17
    \$\begingroup\$ An "intermittent spark" cannot be DC. \$\endgroup\$
    – BowlOfRed
    Commented Aug 18, 2020 at 21:04
  • 13
    \$\begingroup\$ 1. To answer the question in your title: The same way a DC hammer makes a bell ring with AC sound. 2. If you're generating enough AC with a spark gap to excite a neon bulb, you're most likely also generating enough to violate your local broadcasting laws (assuming you don't live in Somalia or someplace where anything goes). \$\endgroup\$
    – The Photon
    Commented Aug 18, 2020 at 21:12
  • \$\begingroup\$ Perhaps the voltage is constant (e.g. your 5VDC or 24VDC supplies), but the current is most definitely not. Your question made me think more about the term "DC" we use all the time meaning direct current, but that doesn't mean the current is constant at all, just that the voltage is. \$\endgroup\$ Commented Aug 20, 2020 at 7:37
  • \$\begingroup\$ In AC the electrons are constantly accelerating and decelerating... which is required to generate EM waves. The constant flow of electrons (DC) creates an EM field (that's high-school electronics). That field is modulated when the flow of electrons changes rate (such as via a spark). You might be confusing a modulated frequency with what the early scientists were experimenting with using sparks. \$\endgroup\$
    – JBH
    Commented Aug 20, 2020 at 16:14

5 Answers 5


We usually use the math of sine waves ("AC") to analyze electromagnetic radiation, and there are good reasons for this. However, the first treatment of how charge generates radiation that my generation of MIT physics students encountered was different.

Consider an isolated charge, not accelerating. Its electric field lines simply point away from it uniformly. Now, move it to one side suddenly. Afterward, its electric field lines still point away uniformly, but from a different place. It's not hard to show, using Maxwell's equations, that the boundary between the space where the field lines originate from the first position and the second will expand as a sphere at the speed of light. On the boundary, the field lines are kinked. That's an expanding, non-sinusoidal electromagnetic wave.

Your spark gap isn't exactly this, but it is closely related: a bunch of charge suddenly moves across the gap. The return current through your power supply is a complication, but the basic result, an impulsive non-sinusoidal electromagnetic wave, is similar.

  • \$\begingroup\$ Upvoted for being an interesting, and entirely different, way of looking at the same phenomenon. And a good explanation on top of it. \$\endgroup\$ Commented Aug 19, 2020 at 18:47

The spark, unless it is a continuous steady arc, will fluctuate in intensity. This means it will have an AC component even if the current never goes in reverse. In practice sparks are very noisy and will generate frequencies over a broad spectrum.

  • 3
    \$\begingroup\$ In reality even in a vacuum, they are never steady due to thermionic noise \$\endgroup\$ Commented Aug 19, 2020 at 11:55

It is not the DC part itself. It is the raising edge. The spark creates a square-like waveform which actually consists of infinitely many sine waves. That is explained by the Fourier transformation. Have a look at the following image to see how many sine waves can create a square one.enter image description here

  • 1
    \$\begingroup\$ This is only a minor factor. As Transistor says, the current will rapidly stop and start while a spark is "firing". This effect can be enhanced by adding an induction coil in series. \$\endgroup\$
    – Hot Licks
    Commented Aug 20, 2020 at 0:24

Neither one of your "DC" sources is producing a steady direct current across the gap. They may produce current only in one direction, but it will be going on and off.

The cause of this is that a spark gap has two states with very different resistances, and behaves in a way that turns a "DC" source into an oscillator:

  1. With no spark, the resistance between the electrodes is extremely high. Essentially no current will flow until the voltage between the electrodes gets high enough to ionize the gas between them.
  2. When the gas ionizes (visible as the spark), it creates a conducting path between the electrodes. The resistance between the electrodes instantly becomes extremely low. The current instantly becomes very high, and this fast transition emits EM radiation.
  3. It would take an incredible amount of power to maintain your supply's very high output voltage across such a low resistance. It doesn't have that kind of power, so after the initial spark, the current soon drops too low to sustain the spark, and it goes out. Usually there is an output capacitor that the spark will drain.
  4. Now we're back to (1). Resistance is high, no current flows, so the voltage can rise until the spark reforms.

When you use your "high power" DC generator, the spark might look continuous, but it's actually switching on and off very quickly.


The effect is most likely (I would think) from the votlage across the neon bulb from an electric field. Could be magnetic (inductive) but I think that would be more difficult with a neon bulb since there isn't much metal.

Could be either of these things:

  • A spark
  • Bleeding from the HV supply into the air (capacitive coupling)

A spark is simply the moment of electrons, moving electrons create magnetic fields. If the field is changing rapidly it creates oscillations of electric and magnetic waves otherwise known as radio waves.

The air (and any metal surface) with electric fields traveling between them can function like a capacitor

The change might also be coming from an HV supply if the coils are unshielded (magnetic fields)

  • \$\begingroup\$ Leakage from the HV supply is a possibility - one could test this by moving the spark away from the generator with longer wires, and then moving the neon bulb around to see where it lights the brightest. \$\endgroup\$
    – jpa
    Commented Aug 19, 2020 at 7:03

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