By assuming the weak lens focal length expression to be the first term in a power series a simple and accurate formula can be derived for the objective focal length of a magneticelectron lens.
Magnetic coils focus the electron beams into fine spots.
磁线圈将电子束聚焦成细小的点。
《柯林斯英汉双解大词典》
3
We have placed the nanotubes between magnetic electrodes and we have shown that the direction of a single electron spin caught on the nanotube can be controlled directly by an electric potential.