E/M (Energy/Mass) by Thomson Method is a technique used to determine the charge-to-mass ratio of an electron, often denoted as e/m. This method is based on the experimental setup of an electron beam that is subjected to both electric and magnetic fields, and the motion of electrons in these fields is studied to determine their charge-to-mass ratio. The Thomson method, named after J.J. Thomson, who first measured the ratio in the late 19th century, is one of the key experiments in the study of electron properties.