from visual import *

print """
Drag or click to show the magnetic field interactively.
Mark the magnetic field vector at the end of the drag.
"""

scene.width = 600
scene.height = 600
Rbig = 0.6
L = 2*pi*Rbig
Rsmall = 0.2
k = 1E-7 # mu-zero/4pi
I = 1.0
Ncoils = 40
Bscale = (2.*Rsmall)/(4*pi*1E-7*Ncoils*I/L)

dphi = 2.*pi/Ncoils/15.
phi = arange(0,2*pi+dphi,dphi)
helix=curve(x = Rbig*cos(phi)+Rsmall*cos(Ncoils*phi)*cos(phi),
            y = Rbig*sin(phi)+Rsmall*cos(Ncoils*phi)*sin(phi),
            z = -Rsmall*sin(Ncoils*phi))
helix.color = (1,0.7,0.2)
helix.radius = 0.01

delta = helix.pos[1:] - helix.pos[:-1]
center = (helix.pos[:-1] + helix.pos[1:])/2.
scene.range = 1.3*(Rbig+Rsmall)
vwidth = L/100

def BField(obs):
    r = obs-center
    rmag = mag(r)
    rmag.shape = (-1,1)
    return sum(k*I*cross(delta, r)/rmag**3)

Bvector = arrow(axis=(0,0,0), shaftwidth=vwidth, color=(0,1,1))
drag = 0

while 1:
    if drag:
        newobs = scene.mouse.pos
        if newobs != obs:
            obs = newobs
            Bvector.axis = Bscale*BField(obs)
            Bvector.pos = obs
    if scene.mouse.events:
        m = scene.mouse.getevent()
        if m.drag:
            drag = 1
            obs = None # force update of position
            scene.cursor.visible = 0
        if m.drop:
            drag = 0
            scene.cursor.visible = 1
            arrow(pos=obs, axis=Bscale*BField(obs), shaftwidth=vwidth, color=(0,1,1))
        elif m.click:
            arrow(pos=m.pos, axis=Bscale*BField(m.pos), shaftwidth=vwidth, color=(0,1,1))