Thomson's experiment

A forum designed mainly for high school physics students in New South Wales, Australia.
Posts: 1
Joined: Tue Oct 21, 2008 9:48 pm

Thomson's experiment

Postby Giles » Tue Oct 21, 2008 10:00 pm

I'm just wondering what Thomson actually did...

The syllabus doesn't specify how Thomson found the charge/mass ratio, but all textbooks and other sources seem to say that after applying both the electric and magnetic fields he applied only the magnetic field. However, his Nobel lecture according to ... ecture.pdf suggests otherwise.

Did Thomson change his mind (and possibly redo the experiment) before 1906, or has the experiment been 'changed' by teachers for educational purposes with circles being somewhat easier to discuss than parabolas? Or is there another explanation?

Thanks in advance,


User avatar
Posts: 745
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Postby joe » Thu Oct 23, 2008 9:26 am

Gday Giles

JJ Thompson did a number of experiments with electrons, of which three are particularly important.

i) show that a cathode beam is bent by magnetic fields
ii) show that a cathode beam is bent by electric fields
iii) determine e/m for the electron.

(iii) was a null method: apply E in one direction and B in the other, then adjust their ratio until the beam is undeflected.

You want to know the order of these. I don't know, sorry.

The Wikipedia article:
seems to put them in the order I list above, but is not clear on the dates. You'll have to look further.

I'll leave this for you to do. I'm a physicist, not an historian. And I'm very sorry (not to mention angry) that your high school physics syllabus has a disproportionate emphasis on history, and very little physics.


Return to “High School Physics”

Who is online

Users browsing this forum: No registered users and 1 guest