//example-22.3
//page no-660
//given
//critical magnetic field
Hc=7.9*10^3  //A/m
//diameter of aluminium wire
d=1*10^-3   //m
//critical current is give by
Ic=2*(%pi)*d*Hc  //A
printf ("the critical current which can pass through a long thin superconducting wire of aluminium is %f A",Ic)