text: A good illustrative device could be the electricity meter you certainly have at home too. It's also referred to as 'electromechanical induction meter' or 'Ferraris meter'.\n\nhttp://en.wikipedia.org/wiki/Electric_meter#Means_of_operation\n\nThe invention of its principle is attributed to Galileo Ferraris:\n\nhttp://en.wikipedia.org/wiki/Galileo_Ferraris\n\nHis prototype would have looked like this:\n\nhttp://www.museodifisica.unito.it/schede/scheda00000010.html