Wednesday, October 6, 2010

JJ Thomson's Experiments with Cathode Ray Tubes

Joseph John (JJ) Thomson was born in Cheetham Hill, England in 1856.  He studied at Cambridge University where he was given the title Cavendish Professor of Experimental Physics as well as Honorary Professor.  In 1906, he won the Nobel Prize in Physics "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases."

A cathode ray tube can be defined as a phosphor-coated glass tube from which the air has been removed.  It contains two electrodes, a cathode which is the negative electrode, and an anode which is the positive electrode.  When a high voltage electrical current is applied between the electrodes, a cathode ray travels from the cathode to the anode.  The interaction of the cathode ray with the phosphor-coated glass tube produces green light, the visible cathode ray.

Experiment 1:  JJ Thomson’s objective in his first experiment was to prove that the rays emitted from the cathode were inseparable from their negative charge.  In his first experiment, JJ Thomson built a cathode ray tube with a metal cylinder on its end, containing two slits that led to electrometers.  These electrometers were used to measure electric charges in miniscule amounts.  While conducting this experiment, Thomson realized that when he applied a magnetic field across the tube, the electrometers did not measure any charge from the cathode ray.  From this result, he concluded that the cathode ray was charged and had been deflected by the magnet showing that the charges and rays were intertwined and inseparable.

Experiment 2:  JJ Thomson tried to prove that the cathode rays carried a negative charge.  Thomson improved the quality of his cathode ray tube and its vacuum.  On the tube he placed metal plates, one positively charged, and the other negatively charged.  The rays were deflected by the electric plates.  The positively charged plate attracted the rays, and the negatively charged plate repelled the rays.  From these results, Thomson was able to conclude not only that charges were bound to the rays, but also that the rays consisted of negatively charged particles, “corpuscles,” later renamed electrons.

Experiment 3:  Thomson wanted to learn more about the characteristics of the electrons.  Thomson measured the charge and mass of the particles by determining how much the cathode rays were bent by electrical currents of varying strengths.  From his third experiment, Thomson learned that the charge to mass ratio of the particles was very large.  This result meant that either the charge of the particle was very large, or the mass of the particle was very small.  He deduced that the electron’s mass was very small, and that the electron was part of the atom itself. 

The electrons discovered by Thomson carry a charge of (–1) and have 1/2000 the mass of a hydrogen atom.  The electron was the first discovered subatomic particle.  Also, no matter what metal was used for the cathode, the rays always traveled in straight lines, could be deflected by magnetic fields, and had the same properties.  Thomson was surprised because this meant that atoms could be broken up into smaller parts, contrary to Dalton’s theory.  

Works Cited 
Cath7.jpg. N.d. Reich Chemistry. Tangient, 2010. Web. 6 Oct. 2010. <‌Fall.2008.MMA.Cushman.Hutchinson.Timeline>.

cathode2.jpg. N.d. rwshocker. N.p., n.d. Web. 6 Oct. 2010. <‌chem_html/‌atomic_stuct.html>.

CRTdrawing.GIF. 1897. J.J. Thomson’s Cathode Ray Tube. N.p., n.d. Web. 6 Oct. 2010. <‌AtomicStructure/‌Disc-of-Electron-Images.html>.

Dchummer. Cathode Ray Tube. YouTube. N.p., 1 Oct. 2008. Web. 6 Oct. 2010. <‌watch?v=O9Goyscbazk>.

Ebbing, Darrell D., and Steven D. Gammon. General Chemistry Sixth Edition. Boston, Massachusetts: Houghton Mifflin, 1999. Print.

Masterton, William L., and Cecile N. Hurley. Chemistry Principles and Reactions Third Edition. Fort Worth, Texas: Saunders College, 1997. Print.

Shuttleworth, Martyn. “J.J. Thomson’s Cathode Ray Experiment.” Experiment Resources. N.p., n.d. Web. 6 Oct. 2010. <‌frequently-asked-questions.html>.

Silberberg, Martin S. Chemistry the Molecular Nature of MAtter and Change. Boston, Massachusetts: McGraw Hill, 2003. Print.

thomson.jpg. Fall 2010. Reich-Chemistry. Tangient, n.d. Web. 6 Oct. 2010. <‌Fall.2008.MMA.Rowe.Timeline>.

Wilbraham, Anthony C., et al. Prentice Hall Chemistry. Boston, Massachusetts: Pearson Prentice Hall, 2008. Print.