1. A device, consisting of four electrodes arranged in alternating polarity, which focuses a beam of charged particles.
2. An instrument for focusing beams of charged particles which has four electrodes with alternately positive and negative polarity; used in electron microscopes and particle accelerators.
3. An apparatus that uses four electrodes set in an alternating positive-negative polarity series to focus the beams of charged particles employed in electron microscopes and particle accelerators.