J. J. Thomson

sec6_2c.gif (19481 bytes) J. J. Thomson (left) and Ernest Rutherford (right)

The Discovery of the Electron (J. J. Thomson)

In 1897, J. J. Thomson found that the cathode rays can be deflected by an electric field, as shown below. By balancing the effect of a magnetic field on a cathode-ray beam with an electric field, Thomson was able to show that cathode "rays" are actually composed of particles. This experiment also provided an estimate of the ratio of the charge to the mass of these particles.

In the SI system, charge is measured in units of coulombs. By definition, one coulomb is the charge carried by a current of one ampere that flows for one second: 1 C = 1 amp-s. When Thomson's data are converted to SI units, the charge-to-mass ratio of the particles in the cathode-ray beam is about 108 coulomb per gram.

Thomson found the same charge-to-mass ratio regardless of the metal used to make the cathode and the anode. He also found the same charge-to-mass ratio regardless of the gas used to fill the tube. He therefore concluded that the particles given off by the cathode in this experiment are a universal component of matter. Although Thomson called these particles corpuscles, the name electron, which had been proposed by George Stoney several years earlier for the fundamental unit of negative electricity, was soon accepted.

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The cathode rays also can be delected by an electric field in a direction which suggests they are negatively charged.

The Raisin Pudding Model of the Atom (J. J. Thomson)

Thomson recognized one of the consequences of the discovery of the electron. Because matter is electrically neutral, there must be a positively charged particle that balances the negative charge on the electrons in an atom. Furthermore, if electrons are very much lighter than atoms, these positively charged particles must carry the mass of the atom. Thomson therefore suggested that atoms are spheres of positive charge in which light, negatively charged electrons are embedded, much as raisins might be embedded in the surface of a pudding.  At the time Thomson proposed this model, evidence for the existence of positively charged particles was available from cathode-ray tube experiments.

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Thomson's Raisin Pudding Model



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