Physics Demo Number: 014

Approximate Run Time: 5 min

Crooke’s Tube and Magnet

Demo Description

Deflection of an electron beam in a permanent magnet field.

Scientific Principles

Motion of a charged particle in a magnetic field

Equipment

• Crooke's Tube

• Solid-state power supply

• Magnet

Equipment Location

• The Crooke’s Tube and Magnet are in Kit (014) on [B-3-2]

• The solid-state power supply also lives in Kit (014). Note: Please handle Kit (014) and contents very carefully. New Crooke's Tubes are difficult to acquire.

• Magnet is also in Kit(014)

Instructions

The photo shows the equipment for demonstrating the electromagnetic force rather convincingly in class.

One should use the solid state power supply instead of any older automotive ignition spark coils, since the newer supply was designed to limit electron-current in the tube and hence reduce the radiation dosage to the demonstrator from the X-Rays arising from collisions of electrons with tube materials.

Switch the wires or the polarity switch on the power supply to make the left end of the tube negative for proper operation. Use only enough voltage to see the beam clearly. There is a horseshoe magnet for vertical deflection of the electron beam.

As various electrons move through the internal horizontal slit in the tube and collide with the fluorescent screen in the tube, one obtains a visual record that the eye interprets as “seeing” an electron beam.

The magnet can be used to deflect moving electrons up or down and thus move the beam up or down.

A new Crooke's Tube has been put into service in July 2010  !!

Writeup created by David A. Burba

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