Discovery of an electron

Source:  Discovery of an electron    Tag:  diagram of cathode ray tube
My previous post about the nuclear atom and its parent experiment occurs later than one I discuss in this post. Before the nuclear atom was  proposed in 1907, the electron was discovered.

This experiment rests on a fundamental understanding of attraction and repulsion between charges. Naturally it is logical to assume the positive and negative charges attract one another while the like charges (positive/positive and negative/negative) repel one another. Here is a diagram of that basic concept:

Millikan used an apparatus called a cathode ray tube. I've always found that apparatus very conceptually confusing. Although I understand it now I'd rather gloss over the actual appartus and focus on the concept of what he did with it.

Millikan basically used the concept of electricity to prove that electrons were negatively charged. He pointed a beam of electricity in a way that it could either bend toward a positively charged area or a negatively charged area. The beam was attracted to the positive plate. This proved that the electron had a negative charge.

Millikan was able to derive a charge/mass ratio from this experiment. Here is the apparatus.