Mass-to-charge Ratio - Charge-to-mass Ratio - The Electron

The Electron

The elementary charge-to-electron mass quotient, is a quantity 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 . 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.

The 2006 CODATA recommended value is = 1.758820150(44)×1011 C/kg. CODATA refers to this as the electron charge-to-mass quotient, but ratio is still commonly used.

There are two other common ways of measuring the charge to mass ratio of an electron, apart from Thomson and Dunnington's methods.

1. The Magnetron Method: Using a GRD7 Valve (Ferranti valve), electrons are expelled from a hot tungsten-wire filament towards an anode. The electron is then deflected using a solenoid. From the current in the solenoid and the current in the Ferranti Valve, e/m can be calculated.
2. Fine Beam Tube Method: A heater heats a cathode, which emits electrons. The electrons are accelerated through a known potential, so the velocity of the electrons is known. The beam path can be seen when the electrons are accelerated through a helium (He) gas. The collisions between the electrons and the helium gas produce a visible trail. A pair of Helmholtz coils produces a uniform and measurable magnetic field at right angles to the electron beam. This magnetic field deflects the electron beam in a circular path. By measuring the accelerating potential (volts), the current (amps) to the Helmholtz coils, and the radius of the electron beam, e/m can be calculated.