Thomson continued

At first, it seemed obvious that the electricity was producing some sort of light that traveled the length of the tube.

This was the obvious conclusion since it traveled in straight lines (it could cast shadows). However, things were not that simple. It was discovered that a magnet held close to the tube would move the shadow. This meant that it was not light (since light is not affected by a magnet).

It was then, that a most remarkable tube was invented. This was similar to a Crookes Tube, but contained a small paddle-wheel on a pair of parallel bars.

When the charge was applied in this case, the paddle wheel traveled across the tube.

The implication was clear to Thomson. In order to push the paddle wheel, the beam traveling across the tube must have mass, that is, it must be made of particles. Since it was affected by a magnet, it must be charged and since it traveled from the negative end to the positive end (we know by where the shadow is) it must be negatively charged. (For the record, this isn't quite true. What's actually occurring requires some serious physics, which is beyond the scope (and intent) of this text.)

Thomson made various tubes using different metals for the negative pole, different types of glass and even filled the tube with different gases before evacuating it (in case trace amounts of the gas left behind were the cause of the beam). He found that no matter what he did he always say the same beam with the same properties.

He determined that these negative particles must be a part of matter and must be smaller than atoms. (We know that they can't be made of atoms since atoms are neutral and no number or combination of neutral things will make a negative.) He named these particles - electrons.

Thomson's Model of the Atom...