Simple Nature - Light and Matter

Thomson played with the electric and magnetic forces until eitherone would produce an equal effect on the beam, allowing himto solve for the velocity,v =(known constant)(known constant #2).Knowing the velocity (which was on the order of 10% of thespeed of light for his setup), he was able to find the accelerationand thus the mass-to-charge ratio m/q. Thomson’s techniques wererelatively crude (or perhaps more charitably we could say that theystretched the state of the art of the time), so with various methodshe came up with m/q values that ranged over about a factor of two,even for cathode rays extracted from a cathode made of a singlematerial. The best modern value is m/q = 5.69 × 10 −12 kg/C,which is consistent with the low end of Thomson’s range.The cathode ray as a subatomic particle: the indexelectronelectronWhat was significant about Thomson’s experiment was not theactual numerical value of m/q, however, so much as the fact that,combined with Millikan’s value of the fundamental charge, it gavea mass for the cathode ray particles that was thousands of timessmaller than the mass of even the lightest atoms. Even withoutMillikan’s results, which were 14 years in the future, Thomson recognizedthat the cathode rays’ m/q was thousands of times smallerthan the m/q ratios that had been measured for electrically chargedatoms in chemical solutions. He correctly interpreted this as evidencethat the cathode rays were smaller building blocks — he calledthem electrons — out of which atoms themselves were formed. Thiswas an extremely radical claim, coming at a time when atoms hadnot yet been proven to exist! Even those who used the word “atom”often considered them no more than mathematical abstractions, notliteral objects. The idea of searching for structure inside of “unsplittable”atoms was seen by some as lunacy, but within ten yearsThomson’s ideas had been amply verified by many more detailedexperiments.Discussion QuestionsA Thomson started to become convinced during his experiments thatthe “cathode rays” observed coming from the cathodes of vacuum tubeswere building blocks of atoms — what we now call electrons. He thencarried out observations with cathodes made of a variety of metals, andfound that m/q was roughly the same in every case, considering his limitedaccuracy. Given his suspicion, why did it make sense to try differentmetals? How would the consistent values of m/q serve to test his hypothesis?B My students have frequently asked whether the m/q that Thomsonmeasured was the value for a single electron, or for the whole beam. Canyou answer this question?Section 8.1 The Electric Glue 471