I was reading this page which says:

What Edison discovered (and it was promptly dubbed the "Edison effect") was that electrical current doesn't need a wire to move through. It can travel right through a gas or even a vacuum.

Maxwell introduced the term displacement current for explaining how the capacitor is passing an alternating current through its plates.

But what I understand is that the displacement current does not move in the form of electrons, but it moves/passes in the form of circulating magnetic fields up to the other side of the plate.

The paragraph claims that the electrical current can move in vacuum. But it doesn't state the form of the current.

For the Edison effect in vacuum, is the current moving in the form of actual electrons or magnetic fields?


In Edison effect in vacuum, is the current moving in the form of actual electrons or magnetic fields?

The current that Edison was observing was due to an electron current with electrons moving through a vacuum. The effect in vacuum tubes is from one side of the tube being heated to thermally excite electrons off of a metal plate, where they can transfer charge to another metal plate. So to answer your question the charge was due to actual electrons.

This is different than the fields Gauss was noticing, because he was experimenting with capacitors, the displacement current is due to electric and magnetic currents changing in the capacitor.

  • \$\begingroup\$ I see. So the current that Edison was observing can also be created by DC voltages. On the other hand displacement current does not exist for DC voltages i.e static electric fields. \$\endgroup\$ – atmnt Oct 28 '17 at 17:38
  • \$\begingroup\$ Yes, that is why vacuum tubes can 'pass' DC and capacitors block DC. The displacement current is zero if there is no 'AC' or potential difference between two metal plates. \$\endgroup\$ – Voltage Spike Oct 28 '17 at 17:45
  • 3
    \$\begingroup\$ @user134429 - I suspect you're rather young - too young to have experience with CRT oscilloscopes and televisions. In both of those cases, the bright spot on the screen is caused by a beam of electrons produced at a hot filament and accelerated to impact the phosphor screen. \$\endgroup\$ – WhatRoughBeast Oct 28 '17 at 18:14

You're thinking of capacitors. The Edison effect is something you do not want in capacitors, and indeed is the major limitation of vacuum capacitors.

In a capacitor, you have an electric field between the plates. This passes AC, but blocks DC, all without allowing electrons to pass between the plates.

As you increase the voltage, though, eventually you get to the point where the electric field is enough to pull electrons from one plate to the other, and suddenly you can pass DC current.

The threshold for this effect varies depending on the geometry and temperature of the cathode, among other things. "Spikes" lower the threshold, as does higher heat. Indeed, at a high enough temperature (ballpark of 1000K) thermal energy alone is enough to 'cast' electrons out into the vacuum.

(I'm glossing over things, here. Thermionic emission actually happens at all temperatures, it's just usually a very small effect.)

The 'Edison effect' usually refers specifically to thermionic emission.

(As an aside, you can actually directly harvest energy from this effect, albeit inefficiently. If you have a hot cathode and cold anode, it will actually generate a (small) potential difference. TANSTAAFL, though, as it's just another heat engine.)

(As another aside, a spark gap even in a vacuum has interesting behavior - namely it still has negative resistance. Electrons leaving the cathode tend to collide with other atoms in the cathode before they leave, heating the cathode. Which causes more thermionic emission. Which causes more electrons to leave. Repeat.)


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