Section 1: Thompson

Discovery:
The electron was discovered by Thompson in 1897. Before this people believed, in more or less the same style as the Greek philosophers, that the fundamental unit of matter was the atom. It was indivisible, and its properties caused the differences between elements that could be observed. People knew about electrical charge, however it hadn't been reduced, much like today people know of gravity, but have yet to reduce it.

It was discovered by physicists messing around trying to invent the florescent bulb, as it was noticed that at the positive end of their bulbs the glass was glowing. It was explained in terms of charged rays being emitted from the negative side of the bulb, being attracted towards the positive, and there causing it to glow. Due to the negative end being called a cathode, these rays were known as "Cathode Rays".

Thermionic Emission:
These cathode rays were being emitted as the filament in the bulbs heated up and electrons had enough energy to escape the material. Normally they bounce up, cool down by releasing photons, and quickly fall back into the metal again, in a process called 'Thermionic Emission'. This is what you observe when you look at red hot metal. In the apparatus they were using however these boiled off electrons were accelerated across the bulb towards the anode by their charge.

Charge to Mass Ratio:
Thompson naturally enough wanted to learn more about these rays. He created an experiment in which a steady stream of electrons were boiled off and accelerated through a hollow and positively charged tube. This stream then travelled between two charged plates. Naturally the negatively charges electrons were attracted up, towards the positive plate. At the same time, however, Thompson placed a pair of magnets perpendicular to the charged plates. As a charge moves through a magnetic field a force is produced on it, and in this experiment Thompson had the magnets accelerating the stream downwards, in the opposite direction to force caused by the plates. If he carefully adjusted the voltage between the plates he could make the beam travel in a straight line. So?
People had previously worked out the formulas for the forces on a charged particle as it moved through a magnetic field and for the force on a charged particle in a magnetic field. The forces were equal to each other:
Electric Force up = Magnetic Force Down.
If we put in the equations for forces that they knew at the time:
(Charge on an electron) x (Potential difference between the plates) / The distance between them) = (Magnetic field strength) x (Charge on the electron) x (Velocity the electron was travelling at)

So what? Well, to start with the charge can be cancelled from both sides. Since you know the d and V, and B can be worked out as well, then the you can re-arrange the formula to find something out!
(Velocity) = (Potential Difference between the plates) / ((Magnetic Field Strength) * (Distance between plates))

It's the speed of the electrons, but how is this useful? Well, the kinetic energy put into something can worked out with the formula:
(Change in Kinetic Energy) = ½ x (mass) x (velocity)2

Now then, there is a formula for the energy of a charged particle in an electric field:
(Energy) = (Charge) x (Potential difference)

Since all the change in kinetic energy must have come from the accelerating voltage, these two types of energy must be equal to each other:
which can be rearranged:


So, by a bit of algebra you can work out the charge to mass ratio of these cathode rays from what Thompson knew at the time. However the charge to mass ratio is not that immensely useful in itself. To find out the rest required the patience of another man, Milikan, who I shall move onto later.