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. 

It may possibly be c i think |-/