Why is this high amperage metal melter making an electric arc?

Question

There is a video on youtube with a device for melting metal (video below). It's a step-down transformer that transforms 120VAC in primary coil to ~3VAC in secondary coil, therefore stepping up the current 40x.

However, I don't understand why it makes electric arc you can see in the video at time 0.10-0.14. Until today I was under impression that arcs are only formed from very high VOLTAGE, not amperage. Is this assumption wrong?

In my mind, this device should simply heat up metal on contact, without the arc.

https://www.youtube.com/watch?v=JIlZsuRc9jQ

Answer 1

The assumption is wrong.

There are two parts to consider with an arc

1. The formation
2. The maintaining

To form an arc two opposite polarity conductors are needed & a sufficiently high enough voltage to initiate a current flow. This is either via ionisation of a gas between the two conductors or via electrons being liberated from the surface of the conductors

Once a current path is established via these ions an arc is formed. This arc will persist for as long as there is sufficient power behind it & a current path exists.

There are a number of things that influence what distance is needed to spontainously form an arc

1. Pressure. Paschen's Law
2. Distance
3. Voltage

Likewise the medium (vacuum, air, gas) and whether the conductors are pointy (concentration of electrons) or spherical.

What occurred in the video is the individual forced the current to flow via physical contact. With the current path establish an airgap was created that was wide enough that current flow was still possible.

Answer 2

Once you form a plasma, it will conduct orders or magnitude better than air so you also need orders of magnitude lower voltage to sustain it. More current = more plasma, given you did manage to strike the initial arc.

That said, you usually try to wind for a bit higher open circuit voltage and "drop" the arc voltage and limit the current with an indictor in series. Although he's not making a traditional arc welder here so the motives are a bit different, high open circuit voltage is what differs a good and easy to use stick arc welder from one of those early and cheap inverter DC welders with too low voltage to restrike and maintain your arc.