In Thomson's atomic model, why did he not assume that there are positively charged particles embedded in a negatively charged sphere?
J. J. Thomson is recognized with blowing apart a long-held belief that atoms were indivisible through his discovery of electrons while studying the deflection of discharges from cathode ray tubes.  He postulated that the particles formed in this way were small with a very high charge to mass ratio.  At this point he had two pieces of information: (1) an atom has an overall neutral charge and (2) part of the atom consisted of these small negatively charged entities which we now call electrons.  He therefore postulated that the rest of the atom consists of positive charge to balance the negative charge of electrons.  It is believed that the plum pudding analogy stems from his proficiency as an instructor and a desire to relate the atomic model to something his students could understand.