I am beginning to learn chemistry/physics, and I have recently read about JJ Thompson's experiment which led to the discovery of the electron. In every source that I've read, the writers note that Thompson used an anode and a cathode to conduct electricity. In addition, the magnet supposedly had N/S ends. I did not know what a charge was, so I looked it up. But, frustratingly, I always get answers that refer to electrons and protons. That is, things that are negative have more electrons than protons, and vice versa. This gives me no idea as to how Thompson inferred that electrons are negatively charged from his experiment.

My question is, can you define charge without talking about electrons and protons? Thompson didn't know electrons existed, so it seems to me that he must've had some other working definition of charge in order to determine that electrons were negatively charged. What makes an object a cathode or an anode, without referring to protons and electrons? If that question doesn't make sense, is it possible to adapt it just for the sake of understanding the experiment?

  • $\begingroup$ No, he knew he was looking at electrons, even though the name hasn't been picked yet. Millikan's experiment tells us what the minimum amount of charge is, and electric fields will tell us which sign that charge is. The magnetic field tells us the mass from these, and just by comparing with ions of various types, it can be shown that the electron is a subatomic material that is way too light to be an ion. He thus knew he had a new thing. He even knew it was a particle, not a wave. His son later proved it was a wave too, though. $\endgroup$ Jul 20 at 2:39

1 Answer 1


. . . . . . without defining charge via electrons and protons?

At the time of his famous experiment in 1897 Thomson the idea of there being protons and electron did not exist.

The idea of positive and negative charge came from Benjamin Franklin to explain various (electrostatic) phenomena and with the names referring to a surplus of a "fluid", positive (charge) which was transferred between bodies and a lack of the fluid, negative (charge).

Later with the advent of discharge tube which had gas in then under very low pressure it was found that "something" started on the negative electrode cathode) , traveled in straight lines (a shadow could be produced) but could be deflected by a magnetic field, and caused fluorescence of glass and a phosphor.
Hence the name $cathode rays$, starting at the cathode and have light rays like properties in terms of the formation of shadows.

The two main theories current at time of Thomson was that the cathode rays were a new type of particle and that cathode rays were a new type of wave.

The deflection of the cathode rays by a magnetic field meant that once collected by a vessel connected to an electroscope it was shown that cathode rays were negative having the same charge as a glass rod has after it was rubbed with silk.

Thomson was also able to deflect the cathode rays with an electric field and thus measured the mass to charge ratio of the cathode rays showing that as they had mass they must be particles.
Measuring the penetrating power of electrons lead Thomson to infer that the mass of a cathode rays particle was $1/1800$ the mass of the lightest atom, hydrogen.

The discovery of cathode rays and showing they were negative particle with a mass much smaller than that of atoms was a hint that atoms were not indivisible with one of the constituent parts the negatively charge electron.

He then proposed that the atom looked like a plum pudding and to explain the two types of static electricity it must contain as atoms are neutral, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it.
Please note no mention of protons which came with the experiments of Rutherford and others.


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