DECLARE EXTERNAL PICTURE YA
PRINT
PRINT "赤い陽子が作る磁場（３次元）の断面のイメージ"
PRINT "      （ 場の大きさは距離の２乗に反比例します。）"
PRINT
SET WINDOW -5,5,-5,5
SET LINE WIDTH .1
SET LINE COLOR 15
PLOT LINE -5,0;5,0
PLOT LINE -.001,5;-.001,-5
PLOT LINE -5,-5/SQR(3);5,5/SQR(3)
PLOT LINE -5/SQR(3),-5;5/SQR(3),5
PLOT LINE -5,5/SQR(3);5,-5/SQR(3)
PLOT LINE -5/SQR(3),5;5/SQR(3),-5
FOR i=1 TO 7
   DRAW CIRCLE WITH SCALE(i*SQR(2))*SHIFT(0,0)
NEXT i
SET LINE COLOR 1
SET AREA COLOR 4
DRAW DISK WITH SCALE(.2)*SHIFT(0,0)
FOR i=1 TO 5   
   FOR j=0 TO 11
      DRAW YA(1.6/i^2) WITH ROTATE(-j*PI/6)*SHIFT(i*SQR(2)*COS(j*PI/6),-i*SQR(2)*SIN(j*PI/6))
   NEXT j
NEXT i
END

EXTERNAL PICTURE YA(D)   !基本の矢印を描く
SET LINE WIDTH 2
PLOT LINES: 0,0;0.8*D,O
PLOT LINES: 0.8*D,O;.5*0.8*D,-.3*0.8*D
PLOT LINES: 0.8*D,O;.5*0.8*D,.3*0.8*D
END PICTURE