What observation did thomson make to suggest that an electron has a negative electric charge?

J.J Thomson, an English physicist, was conducting an experiment using cathode ray tubes. These are sealed glass tubes practically in vacuum that release a beam of particles. His experiment involved placing 2 oppositely-charged electric plates surrounding the cathode ray. It was then observed that the cathode ray was repelled by the negatively-charged electric plate as it moved towards the positively-charged plate. This means that the cathode ray was made up of negatively-charged particles since it was attracted to the positively-charged plate. He performed this procedure using different metals as electrode and still came up with the same result. This observation eventually led to the significant discovery of electrons. 

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