The story of cathode rays begins in 1855. In that year, Heinrich Geissler invented the mercury vacuum pump. With the pump he could remove almost all of the air from a sealed glass tube. Geissler’s friend Julius Plucker used the pump to evacuate a special kind of tube. Inside the tube were two electrodes. Plucker attached one electrode, called the anode, to the positive terminal of a battery. He attached the other electrode, the cathode, to the negative terminal. He noticed that the glass near the cathode glowed with greenish light. When Plucker held a magnet near the tube, the glowing spot moved. Plucker’s student, Johann Wilhelm Hittorf, put solid objects inside the tube between the cathode and the glow. The objects cast shadows. Hittorf concluded that the cathode was emitting something that travelled in straight lines, like light rays. The German physicist Eugen Goldstein named them “cathode rays.”
The English scientist William Crookes thought cathode rays were streams of molecules that had picked up a negative electric charge. Crookes knew from the laws of electricity and magnetism that a charged particle in a magnetic field would move in a circle. Since a magnetic field caused cathode rays to move in a circle, Crookes reasoned, they must be made of charged particles.
If cathode rays were streams of charged particles, an electric field also should have deflected their path. The German physicist Heinrich Hertz tested this hypothesis. He set a cathode ray tube between two metal plates. One plate was positively charged and the other was negatively charged. Negatively charged molecules should have been attracted to the positive plate. When Hertz connected his tube to the battery, the cathode rays kept going in a straight line. Hertz concluded that the cathode rays were a new kind of electromagnetic wave. Hertz’s student, Philipp Lenard, designed a cathode ray tube with a thin foil at one end. The cathode rays went right through the foil. Since molecules of gas could not go through the foil, Lenard knew that cathode rays could not be charged molecules. He agreed with his teacher that they must be electromagnetic waves.
Then Jean-Baptiste Perrin conducted a very simple but very clever experiment. He accelerated a beam of electrons in a glass tube. You can see at the start of my video how the spot on the glass tube is the impact of the electrons causing fluorescent on paint on the inside of the tube. He then setup a magnetic field at 90 degrees to the beam using coils of wire (Helmholz coils). As you increase the current flow inside the coils the field becomes stronger causing the beam to curve according to Flemings LH rule of FBI. Now as the beam is directed down to a collector which is connected to a gold leaf electroscope the leaf rises. This shows us that the beam is in fact charged. Further experiments show the charge is also negative. This is evidence that cathode rays are in not part of the EM Spectrum.
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