Thomson is known for developing the "plum pudding" model of the atom. this is described as negative particles embedded in a gel of positive charge - much like raisins scattered in pudding. what in his experiment led him to believe that the atom contained negative particles?

J.J. Thomson was known for his experimentation of cathode ray tube which is described as a sealed glass vacuum tube. A high voltage is being applied across two electrodes at one end of the tube, which causes a beam particle to flow from one direction to another. Cathode ray tubes are called as it is because the particle beam or "cathode ray" originates at the cathode. 

Thomson tested the properties of the particles, by placing two oppositely-charged electric plates around the cathode ray. The ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray tube is primarily composed of negatively-charged particles.

He also placed two magnets on either side of the tube and observed that the magnetic field present in the tube deflected the cathode ray. This predetermined the mass-to-charge ratio of the cathode ray particles, which led to the discovery that its mass is smaller compared to that of any known atom. He repeated the experiment with different  metals as electrode materials and found that the properties of cathode ray remained constant.

By then, he was able to form conclusions and one of which is that negatively-charged particles are existential  to counterbalance the charge of an atom and these are suspended in a sea of positively-charged particles which preempt the conceptualization of the PLUM-PUDDING MODEL. This negatively charged particles which are known as electrons, according to him, is present in all elements.