Magnetic field around a Fanselau coil[1] consisting of four round current loops symmetric around their common axis with identical current. This arrangement removes variations in the magnetic field, up to its 5th-order derivative with respect to the center.The inner loops have an axial position z1=±0.29759R1{\displaystyle z_{1}=\pm 0.29759\,R_{1}} and radius R1{\displaystyle R_{1}}. The outer loops have an axial position z2=±0.90673R1{\displaystyle z_{2}=\pm 0.90673\,R_{1}} and radius R2=0.76322R1{\displaystyle R_{2}=0.76322\,R_{1}}. The current through all loops is I{\displaystyle I}. The central magnetic field is B0=1.23038μ0I/R1{\displaystyle B_{0}=1.23038\mu _{0}I/R_{1}}.
The dots symbolize where the current flows out of the image plane, whereas the crosses are where the current flows into the plane.
Own work
Geek3