http://www.vintagecalculators.com/html/evolution_of_today_s_calculato.html

http://www.ideafinder.com/history/inventions/handcalculator.htm

http://electronics.howstuffworks.com/gadgets/other-gadgets/calculator1.htm

http://scienceray.com/mathematics/the-history-and-evolution-of-the-calculator/

http://www.youtube.com/watch?v=8dDuZqpp-iU

start with abacus