Simple Nature - Light and Matter

that they were indeed electrically charged, which was strong evidencethat they were material. Not only that, but he proved thatthey had mass, and measured the ratio of their mass to their charge,m/q. Since their mass was not zero, he concluded that they werea form of matter, and presumably made up of a stream of microscopic,negatively charged particles. When Millikan published hisresults fourteen years later, it was reasonable to assume that thecharge of one such particle equaled minus one fundamental charge,q = −e, and from the combination of Thomson’s and Millikan’s resultsone could therefore determine the mass of a single cathode rayparticle.k / Thomson’s experiment provingcathode rays had electric charge(redrawn from his original paper).The cathode, C, and anode, A,are as in any cathode ray tube.The rays pass through a slit inthe anode, and a second slit, B,is interposed in order to makethe beam thinner and eliminaterays that were not going straight.Charging plates D and E showsthat cathode rays have charge:they are attracted toward the positiveplate D and repelled by thenegative plate E.The basic technique for determining m/q was simply to measurethe angle through which the charged plates bent the beam. Theelectric force acting on a cathode ray particle while it was betweenthe plates would be proportional to its charge,F elec = (known constant) · q .Application of Newton’s second law, a = F/m, would allow m/qto be determined:mq=known constantaThere was just one catch. Thomson needed to know the cathoderay particles’ velocity in order to figure out their acceleration. Atthat point, however, nobody had even an educated guess as to thespeed of the cathode rays produced in a given vacuum tube. Thebeam appeared to leap across the vacuum tube practically instantaneously,so it was no simple matter of timing it with a stopwatch!Thomson’s clever solution was to observe the effect of both electricand magnetic forces on the beam. The magnetic force exertedby a particular magnet would depend on both the cathode ray’scharge and its velocity:F mag = (known constant #2) · qv470 Chapter 8 Atoms and Electromagnetism