J. J. Thomson's experiment determined the ratio of charge to mass for electrons. He used a setup with crossed electric and magnetic fields, such that only electrons with a specific speed, the ratio of the electric and magnetic fields, passed through it. In an evacuated glass tube, electrons were accelerated from a hot cathode and transformed into a beam by the potential difference between the cathode and the two anodes. The kinetic energy gained by the electrons equals the lost electric potential energy. So, he determined the electrons' speed. The velocity-selected electrons struck the fluorescent screen and made it glow. Then, he solved the two equations to determine an electron's charge to mass ratio. All quantities on the right-hand side are independent of the type of residual gas in the tube, the cathode material, or any other aspect of the experiment, implying that the electrons in the beam are a fundamental constituent of all matter. The most precise calculated value of e/m is 1.759 x 1011 coulombs per kilogram.