The more resistors which are cut out as the driver notches higher, the more power is developed by the traction motors. At around the locomotive switches to a parallel combination, where the two traction motors per bogie are in a series electrical circuit, while the two bogies are in parallel electrical circuit. Eventually, when all resistors are cut out, the locomotive is operating in full-field. The locomotives in a consisted set are switched between series and parallel mode automatically, based on speed, overhead voltage and electromotive force (EMF). The driver can select "series hold" to force some or all of the locomotives in the consisted set to remain in series mode when overhead voltage is low. If nothing is done, some of the locomotives will switch off under low overhead voltage conditions, to prevent damage to resistors, and switch on again when the voltage improves. The table lists the original Class 6E1 number, series and year built for each Class 18E, Series 1 locomotive, as well as the year it was rebuilt to Class 18E. All the table columns are sortable. The Class 6E1 series lineage identification features, as well as the liveries which were applied to these locomotives, are illustrated in the following pictures. Note the external compressed air pipe on the right side of locomotives rebuilt from Class 6E1, Series 6 and 7.