J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles later named electrons.
(Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.)
To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
Thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. The results of these experiments helped Thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discovery-−minusthe mass of each particle was much, much smaller than that of any known atom. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions: the cathode ray is composed of negatively-charged particles, the particles must exist as part of the atom, since the mass of each particle is only approx. one-two thousandth the mass of a hydrogen atom, and these subatomic particles can be found within atoms of all elements.
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. (its description is very similar to plum pudding, a popular English dessert)
Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup."