差动变压器的动画实现，可以改变初级以及次级线圈匝数-Differential transformer animation, you can change the primary as well as secondary coil turns

计算亥姆霍兹线圈的磁场大小及标准偏差，并比对圆形和方形。-Calculate magnetic field s size and standard deviation of Helmholtz coil, compare typical round and square type.