In this work, an attempt is made to evaluate the errors that have to do with the interpretation and construction of graphic representations. Although the students are studying in the second year of technical high school (secondary education), i.e. in schools with an emphasis in technical subjects (post junior secondary), it is observed that they find difficult to handle the graphic representations. This is an attempt to overcome the difficulties that exist by putting the students to work on electrical machines. They read, construct or verify the graphic representations of the electrical machines using electrical instruments with which they carry out measurements and take values. It is evaluated after the end of each activity and also after a suitable interval of time that there is an improvement in the students' abilities (interpretation and construction) in graphic representations. (Contains 13 figures and 7 tables.)