For the Dolter system a conductor cable was laid in a trench between the rails. At intervals a box was fitted between the rails that contained a stud (which protruded about above the road) and a bell crank. A magnet on a passing tram attracted this crank which then moved to make contact between the conductor cable and stud; once the tram moved away the crank dropped away and the stud was no longer connected to the cable. A long skate was suspended beneath each tramcar which was magnetised by electro-magnets and so both operated the cranks and collected the current that both moved the tram car and powered the electro-magnets. A small battery was carried to charge the electro-magnets should the power be interrupted. The negative return current passed through the rails.