Answer :
He discovered the ELECTRONS. which was first defined sub-atomic particle by anyone...
Ok so yeah here is what JJ Thompson added, I got these from my notes in science class, so here.
JJ Thompson discovered electrons '' plum pudding model'', positive sphere (pudding) with negative electrons (plums) throughout.
JJ Thompson discovered electrons '' plum pudding model'', positive sphere (pudding) with negative electrons (plums) throughout.