<P> In 1896, the British physicist J.J. Thomson, with his colleagues John S. Townsend and H.A. Wilson, performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier . Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known: hydrogen . He showed that their charge to mass ratio, e / m, was independent of cathode material . He further showed that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated materials were universal . The name electron was again proposed for these particles by the Irish physicist George Johnstone Stoney, and the name has since gained universal acceptance . </P> <P> While studying naturally fluorescing minerals in 1896, the French physicist Henri Becquerel discovered that they emitted radiation without any exposure to an external energy source . These radioactive materials became the subject of much interest by scientists, including the New Zealand physicist Ernest Rutherford who discovered they emitted particles . He designated these particles alpha and beta, on the basis of their ability to penetrate matter . In 1900, Becquerel showed that the beta rays emitted by radium could be deflected by an electric field, and that their mass - to - charge ratio was the same as for cathode rays . This evidence strengthened the view that electrons existed as components of atoms . </P> <P> The electron's charge was more carefully measured by the American physicists Robert Millikan and Harvey Fletcher in their oil - drop experiment of 1909, the results of which were published in 1911 . This experiment used an electric field to prevent a charged droplet of oil from falling as a result of gravity . This device could measure the electric charge from as few as 1--150 ions with an error margin of less than 0.3% . Comparable experiments had been done earlier by Thomson's team, using clouds of charged water droplets generated by electrolysis, and in 1911 by Abram Ioffe, who independently obtained the same result as Millikan using charged microparticles of metals, then published his results in 1913 . However, oil drops were more stable than water drops because of their slower evaporation rate, and thus more suited to precise experimentation over longer periods of time . </P> <P> Around the beginning of the twentieth century, it was found that under certain conditions a fast - moving charged particle caused a condensation of supersaturated water vapor along its path . In 1911, Charles Wilson used this principle to devise his cloud chamber so he could photograph the tracks of charged particles, such as fast - moving electrons . </P>

How was the charge of an electron discovered