graphite (carbons with free electrons) and Pt(metal) have free electrons that are capable of conducting electricity... but they themselves are inert (in most cases), hence do not participate in chemical reaction... hence they provide a site which red/ox may occur, yet do not participate in the reaction
addint more electrolytes WONT increase the emf... the increase of concentration of the cations/anions in the electrolyte can increase the emf, as there are now more ions to transfer the electrons...
different vairables:
different half cells used
concentration of ions in the half cell (if ions are being reduced or formed)
number of electrons involved (which essentially is just the different half cells used)
