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ChemWiki: The Dynamic Chemistry E-textbook > Development Details > Approaches > Demos > Additional Demos > Crookes' Tube

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Crookes' Tube

Chemical Concepts Demonstrated

  • Cathode rays
  • Electrons

Demonstration

One end of a Crookes' tube is touched with a Tesla coil.
A horseshoe magnet is brought near the tube.

Alternatively, the tube could also be connected to a vacuum pump.

cathode.gif

Observations

The tube emits a glow in a straight line until the magnet is brought near.  The magnet deflects the glow into a bent line.

Explanations

Cathode "rays" are actually streams of negatively charged particles (i.e. electrons).

Cathode-ray tubes contain a pair of metal plates sealed into a glass tube that has been at least partially evacuated.  If the residual pressure of the gas is small enough, the glass at the end of the tube across from the cathode will glow when the tube is connected to a series of batteries.  The rays given off by the cathode can be deflected by a magnetic field in a direction which suggests that these cathode rays are negatively charged.  A solid object placed in the path of the cathode rays casts a shadow on the wall of the tube across from the cathode.  The cathode rays also can be deflected by an electric field in a direction which suggests they are negatively charged.

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Last Modified
10:28, 2 Oct 2013

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