 In the early 1800s, most people thought that the atom, the most fundamental unit of matter, was indivisible. Also a lot was known about electricity, but no one knew what was carrying the electric current. For example, highly charged cathode rays were produced inside vacuum tubes in the mid-1800s, but it wasn't until the late 1800s that anyone figured out what was carrying the charge. In 1897, J.J. Thompson used a mass spectrometer to measure the mass of cathode rays. Here's how it works. From Newton, we know that force equals mass times acceleration. So mass equals force divided by acceleration. Acceleration is a change in an object's speed or a change in its direction. If we exert an exact amount of force on a particle and carefully measure its acceleration, we'll know it's mass. So we fix the particle velocity with an electric field, measure the radius of the resulting curve as it moves through a magnetic field, and use the basic electric magnetic and centripetal force equations to calculate mass. Thompson showed that the rays were made of particles that were around 9.11 times 10 to the minus 28 grams. That's 1800 times lighter than the lightest atom, hydrogen, that had also been measured with mass spectrometers. Therefore the particles were not atoms. He had discovered a new particle that was later named the electron.