# JJ Thomson Discovery of the Electron

• I

## Main Question or Discussion Point

pg. 243 Falconer, I. (1987) Corpuscles, Electrons and Cathode Rays: J.J. Thomson and the Discovery of the Electron. The British Journal for the History of Science (BJHS, 1987,20,241-276). "One of their most important properties is that they are deflected by a magnetic field. This provided strong support for a particle theory" pg. 243

Why would deflection by a magnetic field be evidence for particles as Thomson was before the discovery of the photoelectric effect and Einstein?

Last edited:

Related Classical Physics News on Phys.org
ZapperZ
Staff Emeritus
pg. 243 Falconer, I. (1987) Corpuscles, Electrons and Cathode Rays: J.J. Thomson and the Discovery of the Electron. The British Journal for the History of Science (BJHS, 1987,20,241-276). "One of their most important properties is that they are deflected by a magnetic field. This provided strong support for a particle theory" pg. 243

Why would deflection by a magnetic field be evidence for particles as Thomson was before the discovery of the phone electric effect and Einstein?
And electromagnetic wave is not deflected by such magnetic field. So this rules out the "rays" as being EM radiation. Since at that time one of the description of the cathode rays is that it might be EM radiation, while the other is that it is composed of particles, this evidence then points to the particle picture.

Zz.

Staff Emeritus
2019 Award
Furthermore, it points to something with a constant charge per unit mass. That's easy to explain in a particle model, and hard to explain any other way.

vanhees71
Gold Member
2019 Award
Well, in the case of the electron, it was by chance that it was discovered as "a particle". It's due to the experimental setup of cathode ray tubes that Thomson discovered the particle aspects first. Due to the rest gas you even can literally see "particle trajectories".

For light it's more or less inevitable to discover the wave aspects first. The reason is that it is very hard to prepare single-photon states, and this has become routine only from the mid 1980ies on.

tech99
Gold Member
And electromagnetic wave is not deflected by such magnetic field. So this rules out the "rays" as being EM radiation. Since at that time one of the description of the cathode rays is that it might be EM radiation, while the other is that it is composed of particles, this evidence then points to the particle picture.

Zz.
However, Michael Faraday previously found that a beam of light could be altered by a magnet. But he did not take this as evidence for particles, just saying that he thought light was magnetic.

ZapperZ
Staff Emeritus
However, Michael Faraday previously found that a beam of light could be altered by a magnet..

Zz.

However, Michael Faraday previously found that a beam of light could be altered by a magnet. But he did not take this as evidence for particles, just saying that he thought light was magnetic.
That effect is not a deflection of the beam and can be explained with wave theory only:

--
lightarrow

tech99
Gold Member
That effect is not a deflection of the beam and can be explained with wave theory only:
lightarrow
I do agree with you, but nevertheless, we can say that EM waves are affected by magnetism.
You will have seen that Wiki quote the paper as from Faraday's notebook:-
Faraday's Diary. Volume IV, Nov. 12, 1839 - June 26, 1847 (Thomas Martin ed.). London: George Bell and Sons, Ltd. ISBN 0-7503-0570-3. The diary is indexed by Faraday's original running paragraph numbers, not by page. For this discovery see #7504, 13 Sept. 1845 to #7718, 30 Sept. 1845. The complete seven volume diary is now in print again.

ZapperZ
Staff Emeritus
I do agree with you, but nevertheless, we can say that EM waves are affected by magnetism.
You will have seen that Wiki quote the paper as from Faraday's notebook:-
Faraday's Diary. Volume IV, Nov. 12, 1839 - June 26, 1847 (Thomas Martin ed.). London: George Bell and Sons, Ltd. ISBN 0-7503-0570-3. The diary is indexed by Faraday's original running paragraph numbers, not by page. For this discovery see #7504, 13 Sept. 1845 to #7718, 30 Sept. 1845. The complete seven volume diary is now in print again.
If this is what you are using, then it is irrelevant to what I stated, which was that light is ".... not deflected..." by magnetic field. There is no deflection here. And in fact, the Faraday rotation effect is more of an evidence of the wave nature of light, not particle, i.e. light consisting of electromagnetic wave.

And please note that the scenario asked by the OP is all in vacuum, not in a medium. A zoo of effects and phenomena can be created and found in a medium that you can't do in vacuum (magnetic monopole, anyone?).

Zz.

vanhees71
True. Magnetic monopoles have been observed as quasiparticles in condensed matter theory observing an exotic material named "spin ice". Quasiparticles are of course no elementary particles but quanta of collective excitations of this material. Today no elementary magnetic monopoles have been observed, i.e., as far as we know given this empirical status, the Maxwell equation $\vec{\nabla} \cdot \vec{B}=0$ still exactly holds.
As far as I know the also the deflection of light by a magnetic (or electric) field in the vacuum has not been observed in a real experiment today. This is easily explained by the fact that this is a quantum radiative correction of order $\alpha_{\text{em}}^4$. The related effect (i.e., described to leading-order by the same QED box diagram) of Delbrück scattering of (quasi-)real photons $\gamma + \gamma \rightarrow \gamma + \gamma$ has been recently observed by the ATLAS collaboration in ultraperipheral Pb-Pb collisions at the LHC.