The electron charge-to-mass quotient, − − e / m e , is a quantity that may be measured in experimental physics. It bears significance because the electron mass m e is difficult to measure directly, and is instead derived from measurements of the elementary charge e and e / m e . It also has historical significance; the Q / m ratio of the electron was successfully calculated by J. J. Thomson in 1897—and more successfully by Dunnington, which involves the angular momentum and deflection due to a perpendicular magnetic field. Thomson's measurement convinced him that cathode rays were particles, which were later identified as electrons, and he is generally credited with their discovery.