Wien developed the Wien filter (also known as velocity selector) in 1898 for the study of anode rays. It is a device consisting of perpendicular electric and magnetic fields that can be used as a velocity filter for charged particles, for example in electron microscopes and spectrometers. It is used in accelerator mass spectrometry to select particles based on their speed. The device is composed of orthogonal electric and magnetic fields, such that particles with the correct speed will be unaffected while other particles will be deflected. It can be configured as a charged particle energy analyzer, monochromator, or mass spectrometer.