magnetic blow-out gaps

I would add just a bit to Richard's explaination of the magnetic blow-out 
spark gaps. The mechanism behind this gap is the Lorentz force. Imagine the 
gap current flows in the x direction and the field lines from the magnet 
lie in the y direction. A Lorentz force (see for example: Sears, Zemansky 
And Young) in the z direction is then exerted on the charge carriers in the 
gap plasma, namely the electrons and ions. The magnitude of this 
force (say MKS units) is:


q= charge of electron or ion
v=velocity of the charge
B=magnetic field

Since the ions are usually more than 10,000 times heavier than the 
electrons and move rather slowly, it is the electrons which are "pushed" 
out of the gap by the Lorentz force. Any plasma robbed of its electrons 
ceases to conduct very well-so the gap quenches. 

A variation on the above formula is to find the total force on the a 
segment of the current:


F=force in newtons
I=current in amperes
B=Magnetic Field in Tesla
L=length of current segment

For fun:
A primary tank circuit current of 1000 amps and a gap length of 1mm 
and Quenching field of .1T=1000 Gauss, F=.1N. Or, F is about .02 lbs. 

-Ed Harris