J.J. Thomson conducted a series of experiments using cathode ray tubes that led to the discovery of the electron. In his first experiment, he observed that an electric current caused a beam to move from the negative cathode to the positive anode. His second experiment showed this beam was composed of invisible rays emitted from the negative plate. His third experiment applied an electric field and showed the particles composing the beam were negatively charged. His fourth experiment under changed conditions still produced the same results, allowing him to infer that all substances contain negatively charged particles, discovering the electron.

1. JJ THOMSON EXPERIMENT (1897)

2. CATHODE RAY TUBE

3. EXPERIMENT 1

4. CATHODE RAY TUBE

5. A Cathode Ray Tube
Source of
Electrical
Potential
Metal Plate
Stream of negative
particles (electrons)
Gas-filled
glass tube Metal plate
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

6. Thomson’s Experiment
- cathode Anode +

7. Thomson’s Experiment
- cathode anode +

8. Thomson’s Experiment
- +
• Passing an electric current makes a
beam appear to move from the
negative to the positive end.

9. Thomson’s Experiment
- +
• Passing an electric current makes
a beam appear to move from the
negative to the positive end.

11. INFERENCE????

12. •A stream of particles could be
seen to flow from cathode to
anode.

13. Experiment 2

18. INFERENCE?
• THE CAUSE OF THE
GLOW IS THE INVISIBLE
RAYS EMITTED FROM
THE NEGATIVE METAL
PLATE OF THE
DISCHARGE TUBE.

19. EXPERIMENT 3

20. ON APPLICATION OF
ELECTRICAL FIELD

21. Thompson’s Experiment
• By adding an electric field

22. Thompson’s Experiment
• By adding an electric field

23. Thompson’s Experiment
• By adding an electric field he
found the moving particles
were negative

24. INFERENCE
•THE GLOW IS
PRODUCED BY
NEGATIVELY
CHARGED
PARTICLES.

25. EXPERIMENT 4

26. CHANGED CONDITIONS
BUT
SAME RESULT OBTAINED

27. INFERENCE?
•THERE ARE
NEGATIVELY
CHARGED
PARTICLES IN ALL
SUBSTANCES.

28. THUS, ELECTRON
WAS DISCOVERED

29. JJ THOMSON MODEL OF ATOM-PLUM
PUDDING MODEL

30. THANKYOU