Slide 4 of 18
The first man to show that the atom consisted of parts was J. J. Thompson, shown in the picture at bottom left at about the age when he made his crucial discovery of the electron.
Thompson and others were studying “cathode rays,” in evacuated tubes that were the prototypes of present day TV tubes and computer monitors. In Thompson’s Cathode Ray Tube, diagrammed at upper left, a stream of these rays were emitted from the negative terminal and could be observed as a glowing blue path inside the tube. Thompson thought of using a magnetic field to separate any charge particles in the rays from the rest of it and discovered all of the rays carried negative charge. He could use the magnetic field to direct the beam to an electroscope and collect these negatively charged particles.
He measured the ratio of their charges to their masses, and by measuring their speeds crudely, was able to establish that these particles, we now call electrons, were much less massive than atoms.