What experiment revealed that atoms possess electrons, negatively charged particles, in their structure?

A negatively charged plate repels a cathode ray, which is generated by placing a high voltage across metal electrodes at opposite ends of a sealed glass tube.

The speed of falling oil droplets sprayed into a chamber was measured with and without voltage. Using X-rays, the oil droplets were given a negative charge and suspended between electrically charged plates. The speed of the oil droplets dropping changes as the voltage varies.

Alpha particles are emitted by a radioactive substance. They are directed at a gold foil, where the majority of them pass through, a few are significantly deflected, and some bounce back toward the radioactive source.