Lorentz Force Demonstrator
by United Scientific

A “fine beam tube” device that projects a sharply focused electron beam through a vacuum containing traces of inert gas. The ionization of that gas around the electron beam creates a glowing discharge, which not only marks the path of the electrons but also helps to maintain sharp focus of the beam. It is capable of showing the deflection of the beam using transverse electric fields as well as magnetic fields of various orientations. The value of e/m can be computed when the beam is bent into a circular path using a homogeneous magnetic field, measuring the accelerating voltage of the electron gun, the strength of the magnetic field and the radius of the circular beam path.

Mounted onto a base that houses all the necessary power supplies and measuring instruments to operate the demonstrator are a vertical vacuum tube and a pair of Helmholtz coils. Surrounded by a light-shielding case, it features a mirror fixed to the back of the case just behind the vacuum tube, and a scale for no-parallax measurements. An additional, detachable panel helps protect the interior of the device when not in use, and binding posts are attached to the rear panel for external multimeters to be used for more precise measurements.

The vacuum tube itself contains an electron gun assembly that has been oriented to project the beam horizontally, and carries a pair of electrodes for electrostatic deflection in the vertical direction. The tube socked is mounted directly on top of the power supply unit and is capable of being rotated 180 degrees around the vertical axis, varying the orientation of the electron beam in relation to the Helmholtz coils.

Permanently mounted atop the power supply unit, the Helmholtz coils feature an adjustable and removable transparent scale, which allow the diameter of the electron beam’s circular path to be measured. The mirror at the back of the case helps to eliminate parallax when trying to determine the electron beam position, while two red LED’s on the coil frame show the direction of the current as it flows through them.

Providing power to the filament, the grid and accelerating voltages, and electrostatic deflection voltages to the electrode assembly, the power supply also produces the current for the Helmholtz coils. Both the accelerating voltages and the Helmholtz coils have adjustable knobs and a built-in panel meters, with the deflection voltage capable of being adjusted by a separate knob. A three position switch controls the polarity of the voltages and the power for the deflecting electrodes and the Helmholtz coil.

A full operator’s manual, complete with experiment descriptions, is included with the unit.

Specifications:

Vacuum Tube:
-Diameter: 160mm
-Voltages: 0-250V (acceleraion), 50-250V (deflection)

Helmholtz coils
-Dimensions: 280mm (mean radius), 140 turns each