School of Physics Lecture Demonstration Manual

Ef-5 The Magnetic Mirror

Published: Tuesday 14 March 2006 - Updated: Tuesday 29 March 2011


To demonstrate the motion of charged particles in a non uniform magnetic field.



Click on pictures to enlarge.

Magnetic mirror pic2

Magnetic Mirror Pic

Magnetic Mirror Diagram 1

Magnetic Mirror Diagram 2

Magnetic Mirror Diagram 3


The double beam tube consists of two independent electron guns mounted in a low pressure helium filled bulb. Connect the tube in the circuit of figure 1. Switch on the heater supply and allow at least one minute for the cathode to reach temperature. Make sure the anode supply is reduced to zero before the heater supply is turned on. Slowly increase the anode voltage until the beam extends to the wall of the tube.

It will be necessary to have all lights out for the demonstration. Slowly bring up the pole of the bar magnet parallel to the beam direction and as close to the tube as possible. The electrons follow a concergent helical path towards the pole near which their motion is reversed and then follow a divergent spiral path away from it (as in figure 2).

Safety notes

Electrical safety 


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