An early type of Siemens' Self Exciting dynamo (1873) with drum-wound armature and bar commutator. Modern generators with field coils are self-excited, where some of the power output from the rotor is used to power the field coils. The rotor iron retains a magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field creates a weak voltage in the stator coils, which in turn increases the field current, until the machine "builds up" to full voltage. The invention of the self-exciting field by Varley, Siemens & Wheatstone removed the need for a magneto exciter. Although the use of magnetos here is now obsolete, separate exciters are still used for high power generating sets, as they permit easier control of output power. These are particularly common with the transmissions of diesel-electric locomotives.