In this work we analyzed how electric and magnetic lines of force were conceived by Faraday and how they are understood by a group of Argentine university students after receiving instruction. Our results show that many students possess ideas similar to those of Faraday in that lines of force are conceived as real physical entities responsible for the transmission of the electric action. These results are in accordance with the interpretation of physical entities as "matter-based" suggested by Chi et al. (1994). It is suggested that emphasis on the geometrical character of these lines seems imperative in basic electromagnetism classes in order to avoid this kind of confusion.