Subject: determing the critical magnetic fields for type II superconductor

Hi

I'm doing a lab report on high temperature superconductors, more specifically 
yttrium-barium-copper-oxide. i read that you can determine the two critical 
magnetic fields from a graph of critical current against magnetic field 
strength by finding the tangent to the curve at the y-intercept and then 
looking at where this line crosses the x- axis. and then the second critical 
value was where the curve intercepted the x-axis. Could you please explain this 
to me as at present i really don't understand. many thanks.

Jonny

Re: determing the critical magnetic fields for type II superconductor