PhysicsLAB: Famous Experiments: Cathode Rays

In 1897, J. J. Thomson was refining experiments that dealt with the glowing paths revealed when currents of electricity provided by high voltage sources passed through evacuated glass tubes. He was to eventually declare that these mysterious "cathode rays" were actually beams of electrons, small building blocks of matter.

Video courtesy of
Chemistry Lecture Demonstrations
University of Illinois at Urbana-Champaign

In the picture above, the cathode is on the left. It is where the electrons originate. The anode, or positive terminal, is on the right and is the electrode towards which the electrons are being accelerated by the electric potential placed across the tube. A metal plate coated by phosphors is positioned inside the tube to detect the path of the electrons. It emits a green glow when struck by electrons.

Scientists had discovered as early as the 1850's that when a magnet was held up next to the cathode ray tube, the beam would be deflected. This result can be seen in the following video.

Cathode Rays
AVI film clip (1.2 Meg)

Refer to the following information for the next two questions.

After you have watched the video, determine whether the side of the magnet closest to the camera for each captured frame would be the North pole or the South pole. Remember that electrons obey a left-hand rule since they are negatively charged!

Although the properties that J. J. Thomson defined for the electron are not quite what we know to be true today, his work with the discharge of electricity in gases earned him the 1906 Nobel Prize in Physics.