File:VFPt anti-helmholtz coil thumb1.svg

# paste this code at the end of VectorFieldPlot 1.3
doc = FieldplotDocument('VFPt_anti-helmholtz_coil_thumb1', width=220, height=200, unit=40, commons=True)
rx, ry = 1.0, 0.5
n=4

field = Field({'ringcurrents':[[0, -ry, pi/2, rx, -1], [0, ry, pi/2, rx, 1]]})

for sx, sy in [(a, b) for a in (-1, 1) for b in (-1, 1)]:
    for i in range(n):
        t = 1 - (1 - 0.79) ** ((i + 1.0) / n)
        line = FieldLine(field, [sx*rx*t, sy*ry*t], directions='both', maxr=10)
        doc.draw_line(line, linewidth=1, arrows_style={'scale':1.5,
            'max_arrows':2, 'offsets':[1.1, 0.25, 0.1, 1.1]})

doc.draw_currents(field, scale=10.0/14.0)
doc.write()