I have a pool of mercury contained within a sealed glass tube. There are two electrodes placed within the tube. The left hand side electrode contacts the mercury. There is a gap of almost 1cm from the right hand side of the mercury pool to the right hand side electrode. A high voltage (approx 10Kv current is applied, the Hv input electrode connected to the left hand side electrode and the ground connection to the right hand side electrode.

An 555 oscillator circuit drives the Hv and the frequency of oscillation can be altered by a 50KOhm pot. A strong Nd magnet is placed alongside the tube, the North pole (of the magnet) is facing the tube. If the oscillation frequency is set at the lowest possible position and no magnetic field present the arc is in a straight line. When a strong magnet is put into the position stated above the arc curves strongly towards the magnet (this is expected of course) and now if the frequency of oscillation in the primary coil is increased slowly the arc can be made to also oscillate in a wide fan-like pattern.

I realise many things can be happening within the tube: namely ionized Hg (there is heating of course above ambient due to the flow of current, those ions must be? under the affect of the magnetic field when present (sort of like a sputter coater) The power supply is 12VDC.

I do not see an oscillation of the pool of mercury as current obviously passes through it and must be setting up a magnetic field? My question is this a kind of Hall Effect or is it a little more complicated. I enclose a schematic of the setup. The footage (not shown) does show (probably some sort of plasma?) moving in small jets towards the magnetic field).

enter image description here

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    $\begingroup$ It would make your question easier to read if you broke it up into paragraphs. $\endgroup$
    – The Photon
    Oct 23, 2019 at 16:17
  • $\begingroup$ Done! Many apologies! Also I would add that I would have expected the curvature to be for the magnetic south pole in this setup for the Hall Effect. $\endgroup$ Oct 23, 2019 at 16:49

1 Answer 1


This isn't the Hall effect. The Hall effect is that you see a voltage across a wire transverse to the direction of current flow (i.e. not along the length of the wire, but between one side of the wire and the other) when there's an applied magnetic field.

The fact that the arc curves in the magnetic field is just the effect of the Lorentz force on the free carriers that form the arc. ${\bf F} = q{\bf E} + q{\bf v}\times{\bf B}$.

now if the frequency of oscillation in the primary coil is increased slowly the arc can be made to also oscillate in a wide fan-like pattern.

If this is the effect you're trying to explain, I expect it's just because as you vary the HV voltage you change the relative effects of the E-field and B-field in determining the trajectory of the carriers in the plasma forming the arc.

  • $\begingroup$ Hi, thanks, yeah I didn't think it was the Hall Effect from what I could remember- hence having the cross tubes on a Hall effect tube, right . Yes this is the effect I was trying to explain, the fanning, as the Hv changes. This would also explain small amounts of plasma being drawn towards the field too I guess. While I can't see this (the plasma moving towards the magnet-it's too fast) with the naked eye I can capture it on video. Thanks very much for that, my physics is very rusty (actually I'm one f those Biologists LOL!). Much appreciated! $\endgroup$ Oct 23, 2019 at 18:49
  • $\begingroup$ Sorry I meant to say "small amounts of plasma being drawn towards the magnet (looks like they are shooting off in small bursts) whilst the arc is oscillating in it's fan-like pattern" $\endgroup$ Oct 23, 2019 at 19:05

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