The oil drop experiment is a classic method utilized in physics to determine the fundamental ratio of charge to mass ($e/m$) for elementary particles. The procedure involves suspending microscopic droplets of oil within a chamber where both a uniform electric field and gravity are present. By carefully measuring the terminal velocity and trajectory of these charged drops, researchers can analyze the balance between gravitational force, electrical force, and aerodynamic drag. The quantifiable results provide crucial data regarding the quantization of electric charge, thereby contributing significantly to the development of modern atomic theory and our understanding of particle physics.