In J. J. Thomson's paper (Phil. Mag. S. 5. Vol. 44. No. 269. Oct. 1897) concerned with cathode rays, Thomson writes, that the experiment by Perrin that supposedly proves that cathode rays are made of negatively charged particles is open to objection. I don't see why, here is his description of Perrin's experiment:

This has been proved to be the case by Perrln, who placed in front of a plane cathode two coaxial metallic cylinders which were insulated from each other: the outer of these cylinders was connected with the earth, the inner with a gold-leaf electroscope. These cylinders were closed except for two small holes, one in each cylinder, placed so that the cathode rays could pass through them into the inside of the inner cylinder. Perrin found that when the rays passed into the inner cylinder the electroscope received a charge of negative electricity, while no charge went to the electroscope when the rays were deflected by a magnet so as no longer to pass through the hole.

The way Thomson describes Perrin's experiment it seems like the cathode rays by default pass through the holes in the coaxial cylinders and fall onto the inner one, charging it. When the cathode ray is deflected by a magnet no charge is collected.

Thomson then describes his own experiment. To me the difference to Perrin's experiment is, that the cathode rays by default hit the glassbulb and the electroscope shows a charge only when Thomson directs the cathode ray onto the cylinder connected to the electroscope with a magnet.

I feel like both experiments are suited to show that cathode rays are made of negatively charged particles. Why is Perrin's experiment open to objection but Thomson's is not?


1 Answer 1


Thomson's objection was that Perrin's experiment left open the possibility that the electric charge is being carried along by the cathode ray and may be separable from it.

The article 3 Experiments 1 Big Idea provides some useful historical background, writing :

Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays.


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