void Bio0(){
 double mu0=1.256637062*1E-6;
 double x[256*40],y[256*40],z[256*40];
 double Hx[100],Hy[100],Hz[100];
 double xm[100],ym[100],zm[100];
 int nm=1;
 int jn=256*40;
 double pi=TMath::Pi();
 double a=10.;
 double I0=1.;//[A]
 for(int i=0;i<nm*1;++i){
    xm[i]=0;
    ym[i]=0;
    zm[i]=0+20*i/(double)nm;
 }
 for(int j=0;j<jn;++j){
    double theta=2*pi*j/(double)jn;
    x[j]=a*cos(theta);
    y[j]=a*sin(theta);
    z[j]=0;
 }
 for(int i=0;i<nm;++i){

     Hx[i]=0; Hy[i]=0; Hz[i]=0;
    for(int j=0;j<(jn-1);++j){
      double ds[3];
      double rr[3];
      double dHx,dHy,dHz;

      ds[0]=x[j+1]-x[j];
      ds[1]=y[j+1]-y[j];
      ds[2]=z[j+1]-z[j];
      rr[0]=xm[i]-x[j];
      rr[1]=ym[i]-y[j];
      rr[2]=zm[i]-z[j];

      double rr0=sqrt(rr[0]*rr[0]+rr[1]*rr[1]+rr[2]*rr[2]);
      dHx=(ds[1]*rr[2]-ds[2]*rr[1])/(double)pow(rr0,3);
      dHy=(ds[2]*rr[0]-ds[0]*rr[2])/(double)pow(rr0,3);
      dHz=(ds[0]*rr[1]-ds[1]*rr[0])/(double)pow(rr0,3);
      Hx[i]=Hx[i]+dHx;
      Hy[i]=Hy[i]+dHy;
      Hz[i]=Hz[i]+dHz;
    }
    Hx[i]=I0*mu0*Hx[i]/(double)(4*pi);
    Hy[i]=I0*mu0*Hy[i]/(double)(4*pi);
    Hz[i]=I0*mu0*Hz[i]/(double)(4*pi);
    printf("xm[%d]=%e ym[%d]=%e zm[%d]=%e\n",i,xm[i],i,ym[i],i,zm[i]);
    printf("Bx=%e By=%e Bz=%e\n",Hx[i],Hy[i],Hz[i]);
    printf("BB=%e\n",I0*mu0/(double)(2*a));
   } 


}