What makes Jacob's ladder spark to go upwards? In a Jacob's ladder (https://www.youtube.com/watch?v=GaC4BnbH1NY&index=3&list=PL_SAbPKia2YMFi-t8z5WFYCd86Plcs1Un) a spark is originated in the bottom and climbs up to the top.
I have read in several places that what makes the spark to go upwards is the heating of the air. I wonder if it could not also be attributed to the magnetic force generated by the currents across the conductors.
What would happen if we put the ladder upside down? Would the spark go upwards or downwards?
 A: Note that a Jacob's ladder is constructed with the electrodes slighty slanted outward away from the voltage source, so the narrowest gap is at the bottom. This lower gap is sized with the voltage source such that breakdown can occur. Breakdown happens by gas molecules ionizing, and once ionized the resistance of the gap plummets. You have a plasma. The smaller distance, ionized gap represents the path of least resistance,  and this is where current initially flows.
But the plasma is very hot, heating the gas above it relative to the gas below it. The hotter gas provides a lesser path of resistance, even though the gap is a little bit wider, so the spark progresses upward. It continues until the width of the gap becomes so wide that resistance becomes too large for current to flow with magnitude that sustains a plasma,  and continued ionization. The spark is extinguished.
And as soon as it's extinguished,  the gap at the bottom once again becomes the lower resistance. A spark jumps, and the cycle is repeated.
You can test this by taking a strong fan and blowing gas across the ladder. The ladder will fail to operate.
If you turn the ladder upside down the spark will jump at the narrowest gap, but will get stuck there since the heating, gradient is also upside down.
A: We just put Jacob's Ladder Upside down and the spark DID NOT go up. So it tells us that the magnetic field doesn't play a significant role in this experiment: https://youtu.be/eOyk8j2ertU. It is hot air moving up (convection of air) and not the magnetic force.
A: The arc is part of a current loop. The magnetic field generated by a current loop tends to force it apart. So the magnetic field should also help the arc to rise. This link says heating is dominant. I haven't tried it myself.
If you have an external magnetic field, it can exert forces on the arc and move it. This can be strong enough to overcome the force from hot air. See this. 
This link explains some details of how a Jacob's Ladder works, and relates to plasma used for fusion reactions. 
A rail gun is a current loop where magnetic forces exert a very large force on a movable link in a current loop. It has a shape much like Jacob's Ladder. Magnetic force is proportional to current. A rail gun is a low voltage, high current circuit. Jacob's Ladder is high voltage, low current. 
