100 Electron
Finally, Thomson built a new cathode ray tube with a thin slit through a metal plate.
Cathode rays were channeled through this narrow slit. Beyond that metal plate he added a
magnetic field to deflect cathode rays in one direction, followed by an electric field that
would deflect them back in the other direction.
Thomson knew the force these two fields created. Once he measured the amount of deflection (change of direction) each force created in the stream of cathode rays, he could calculate the mass of the particles in this cathode ray stream. That would finally solve the
mystery by identifying the specific particles.
He ran his experiment and didn’t believe his results. The ratio of electric charge to particle mass was way too big, and that meant that the mass of these particles had to be much
smaller than any known particle.
He repeated the experiment a hundred times. He ripped apart and rebuilt each piece of
equipment. The results were always the same. The mass of this particle had to be less than
1/1000 of the mass of a proton (a hydrogen atom)—one thousand times smaller than the
smallest atom—supposedly the smallest possible particle.
Thomson had discovered a new particle—the first subatomic particle. It took hundreds
of demonstrations and several detailed articles before anyone believed that his new particles
existed.
In 1891 Irish physicist George Stoney had named the fundamental unit (particle) of
electricity the “electron” without having any idea what that particle was like. Thomson decided to use Stoney’s name (electron) for his new particle since it carried electrical current.
In 1898 a Frenchman named Bequerel found photographic proof of the existence of subatomic particles to confirm Thomson’s discovery.
Fun Facts: If an electron weighed the same as a dime, a proton would
weigh the same as a gallon of milk
More to Explore
Dahl, Per. Flash of the Cathode Rays: A History of J J Thomson’s Electron. Abingdon,
England: Taylor & Francis, 1997.
Davis, E. J.J. Thomson and the Discovery of the Electron. London: CRC, 1997.
Rayleigh, D. The Life of Sir J.J. Thomson: Sometime Master of Trinity College, Cambridge. New Castle, DE: Dawsons of Pall Mall, 1996.
Sherman, Josepha. J. J. Thomson & the Discovery of Electrons. Hockessin, DE:
Mitchell Lane, 2005.
Thompson, George. J. J. T