... to observe such a deflection, which gave support to the view that the cathode rays are not electric particles. Thomson realised that the reason for Hertz's failure was that the gas in his container was not at a sufficiently low pressure. Consequently, positive and negative ions in the gas neutralised the electric field that Hertz was applying. Thomson reduced the pressure and observed a deflection. Thomson's claim to be the discoverer of the electron rests on two key observations. First, he found that the value of e/m was of the order of 1000 times larger than its value for the lightest particle then known, which was the hydrogen ion in electrolysis. On the assumption that the charge was the same for both particles, this meant that the mass of the new particle was of the order of 1000 times less than that of the hydrogen atom. (We now know that it ...

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