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The effect of magnets on cathode rays

Magnets have a dramatic effect on cathode rays. View the following images to trace the story of this effect. Click on the photographs on the right.

Image 1: The demonstation tube

A glass tube with a fluorescent screen placed down the centre.

A cathode ray tube can be used to see the effect of magnets on a cathode ray. Down the centre of the tube is a fluorescent screen to show up the path of the rays.


Image 2: The collimator

A close-up of the collimator which is a metal barrier with only a narrow slit in the centre.

A collimator is placed at the cathode end of the tube. It contains a slit that narrows the beam of the cathode rays.


Image 3: A narrow beam of cathode rays is formed

A glass tube with a blue line glowing along the white fluorescent screen.

This narrows the beam of the cathode rays, as the voltage is increased the beam becomes brighter.


Image 4: Effect of a magnet

A magnet held over the cathode ray tube is cause the beam to deflect away.

The most extreme deflection occurs when the magnet is close to the beam and the poles are facing outwards. If the south pole is facing out the beams is deflected down. When the north pole is facing out the beam is deflected upwards.


Image 5: Aligning the magnets

The magnet is under the cathode tube. The poles of the magnet are aligned up and down.

When the poles of the magnet are facing up and down the beam tapers off at the end and fails to make it to the end of the screen. With the south pole facing up the beam is deflected towards the front of the tube.


Image 6: North pole up

The cathode rays do not reach the end of the tube, The magnet placed underneath with the north pole upwards is deflecting the rays backward.

When the north pole is facing up the rays are deflected towards the back of the screen.


Image 7: Horizontal magnets

The magnet is horizontal underneath the cathode ray tube.

When the poles are horizontal there is little deflection of the rays.


Image 8: Conclusion

A glass tube

In conclusion, a magnet deflects cathode rays, the closer the magnet the greater the deflection.


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