In 1897, J.J. Thompson ,along with a group of his graduate students, set out to investigate this particle. He designed some tubes containing electrodes inside with the air evacuated from the tubes. These were called "Crookes Tubes" named after the original designer. They would later be called "Cathode Ray Tubes". Experiments were performed on these tubes in which high voltage electrical current was passed between the two electrodes. Ray like emanations proceeded from the Cathode electrode to the Anode electrode. Since these emanantions originated from the Cathode electrode they would be called "Cathode Rays". J.J. Thompson designed some special tubes that investigated the properties of these "Cathode Rays". He designed a tube that allowed a beam of these Cathode Rays to impact against the surface of a Zinc Sulfide coated screen. As the rays impacted on the surface, it emitted a spark of light so that the invisible ray's path could be observed. He then proceeded to bring an electrical field consisting of a positive plate and a negative plate near the vacinity of the Rays. When the electrical current of the electrical field was turned on, the path of the "rays" was deflected away from the negative plate and toward the positive plate. This was a clear indication that the so called rays possessed a negative charge. Another Crookes tube design had an object, a Maltese Cross, placed just past the exit path of the cathode rays as they went from the cathode to the anode.
A shadow of the cross was cast upon the front of the tube. The only way that the "rays" could cast a shadow impression on the back of the tube was if they went past the exit path and struck the cross. This would strongly indicate that the rays possessed momentum, but in order for anything to have momentum that would mean that the rays would have to possess mass since
momentum = mass x velocity.
But if the "rays" possessed mass that would mean that they were not rays (pure radiation) at all but particles with a finite mass!! Other tube experiments involving a paddle wheel placed in the path of the cathode rays resulted in the movement of the paddle wheel when the current was turned on. In order for the paddle wheel to be caused to move, the Rays would have to have momentum passed on to the wheel. That would mean that the so called rays would have to possess momentum itself in order to impart momentum to some other object. These "rays" had been termed "electrons" in 1891 by Professor Stony. Prof. Stony investigated electricity as an energy source for chemical reactions. He suggested that electrical current was the result of moving particles that he suggested should be called "electrons".
These experiments definitely defined the rays as actual particles having a negative charge and a finite mass. In 1886, Professor Goldstein performed similar experiments using a perforated cathode surface. This produced a particle that possessed a positive charge and a mass some 2000 times more than Thompson's electron. This particle was called the proton. Since both electrons and protons have come from the surface of matter, it is logical to conclude that all matter is composed of these particles within the atoms of matter. It is interesting to note that the third sub-atomic particle that we know the atom to have was not observed until 1932 some 35 years after the discovery of the electron and the proton. A lot of work had been done during that 35 year period that would not be affected by this belated discovery.
The elusive particle had been predicted in 1920, but it wasn't until 1932 that Professor Chadwick observed these neutral particles that he called neutrons while performing a series of cloud chamber experiments. It was the condensation paths of the neutrons similar to the jet trails that jet engines make when at high altitude that allowed the observance of these elusive particles. Since the key to our understanding of the chemistry of matter resides in our knowledge of the electrons and protons, the belated discovery of the neutrons did not alter the rapidly forming picture of the atom in 1932.
A brief biography of James Chadwick is available.
In 1909, Robert Millikan performed his legendary oil drop experiment which allowed him to determine the exact magnitude of the charge of the electron, 1.60 X 10-19 coulomb. Earlier, Thompson determined the charge to mass ratio of the electron, 1.76 X 108 coulomb / gram, so this determination of the charge by Millikan allowed the determination of the mass of the electron, 9.09 X 10-28 grams.
A brief biography of John Joseph Thompson is available.
R. H. Logan, Instructor of Chemistry, Dallas County Community College
District, North Lake College.
Send Comments to R.H. Logan: Profchm@aol.com
All contents copyrighted (c) 1996 R.H. Logan, Instructor of Chemistry,DCCCD All Rights reservedRevised: 7/26/2001
Original Date of Creation: 11/28/96
URL:http://members.aol.com/profchm/current html filename