Bending a beam of electrons

A cathode ray tube like the one J.J. Thomson used to discover the electron. Change the voltage on the plates and the current in the coils, and watch how electric and magnetic fields push the beam.

Screen, face on

0.0 mmhow far the spot has moved

Grid squares are 1 cm. The screen keeps glowing for a moment after the beam moves.

Controls

6.3 V supply to filament

Speeds up the electrons. Higher voltage means faster electrons.

Positive: the upper plate is positive. Negative: the upper plate is negative.

Positive and negative currents make magnetic fields in opposite directions.

The small buttons change the current by 1 or 10 thousandths of an amp (mA).

Readings

Meters

Accelerating voltage Va
Plate voltage Vp
Coil current I

What the markers mean

Blue arrows between the plateselectric field
⊗ and ⊙ symbolsmagnetic field
Blue arrow on the beamelectric force
Orange arrow on the beammagnetic force

How the model is simplified

  • The fields are shown only between the plates. Real fields spread out a little beyond them.
  • You can see the beam here to make it easier to follow. In a real tube the inside is a vacuum and the beam is invisible. You only see a glow where it hits the screen or the glass.
  • The electrons are shown moving very slowly. Real ones cross the tube in a few billionths of a second.
  • Gravity is ignored, because its effect on an electron is far too small to notice.

In 1897 J.J. Thomson used a tube like this to show that cathode rays are streams of particles we now call electrons. It was the first discovery of a particle smaller than an atom.